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PROCEEDINGS

OF THE

AMERICAN ACADEMY

OF

ARTS AND SCIENCES.

Vou. XLVIII.

FROM MAY 1912, TO MAY 1913.

BOS TON: PUBLISHED BY THE ACADEMY 1913

The Cosmos Press

EDW. W. WHEELER

CAMBRIDGE, MASS. ||

CONTENTS.

PAGE On the Ultra Violet Component in Artificial Light. By Louis BELL : Ἐν Sb On eS Ἐς 1 Alexander Agassiz. By Henry Ρ. Waucotr 31 A Theory of Linear Distance and Angle. By H. B. PxHtnures and C. L. E. Moors Serta aes τ 3) Preliminary Diagnoses of New Species of Chaetomium. By A. H. CHIVERS 81 A Study with the Echelon Spectroscope of Certain Lines in the Spectra of the Zinc Arc and Spark at Atmospheric Pressure. By Norton A. Kent 91 The Impedance of Telephone Receivers as affected by the Motion of their Diaphragms. By A. E. KennEtLyY and G.W. Pierce 111 New or Critical Laboulbeniales from the Argentine. By Routanp THAXTER - 155 Culture Studies of Fungi producing Bulbils and Similar Propaga- tive Bodies. By J. W. Hotson + 225

Thermodynamic Properties of Liquid Water to 80° and 12000 Kgm. By P. W. BripGMan

Preliminary Descriptions of New Species of Rickia and Treno- myces. By Rontanp THAXTER

The Space-Time Manifold of Relativity. The Non-Euclidean Geometry of Mechanics and Electromagnetics. By EH. B. Witson and ἃ. N. Lewis :

On the Existence and Properties of the Ether. By D.L. WEBSTER

The History, Comparative Anatomy and Evolution of the Arau- carioxylon Type. By E. C. Jerrrey

iv CONTENTS. XIV. The Action of Sulphur Trioxide on Silicon Tetrachloride. By C. R. Sancer and E. R. Rirceu ot 3 εν een OS XV. An Electric Heater and Automatic Thermostat. By A. L.CLuarK 597 XVI. Cretaceous Pityoxyia from Cliffwood, New Jersey. By Rutu

FGEDENS em) τα Bole he ices eee ey Ubi ρον mL XVII. On the Scalar Functions of Hyper Complex Numbers. By ἘΠΕ ΝΕ YGe GRABER: 91S oe Wa ed ets PLS Doty ΣΉ 099

XVIII. Preliminary Study of the Salinity ue Sea Water in the Bermudas. ihe ἼἼ | By ΠῚ Marx . Bue eee ail eee Unie ee

XIX. On Certain Fragments of the Pre-Socratics: Critical Notes and Hiuctdations. By W.:A.. HEIDEL: τ. (0 4. 40 a) ee ῸΠ

XX. The Structure of the Gorgonian Coral Pseudoplexaura crassa Wright and Studer, “By W: M.<C@HESTER). 2 τ 4% τ [ὲὺ

XO HR ECORDS) OF) IVERETINGS, τ Ὁ. ἐπι ton ιν δος Τ᾽

BIOGRAPHICAL NOTICES:

oben Amonys “By Re ΕΠ BYEZ. 4, πὸ VY eines es), by OS Abbott Lawrence Rotch. By R. DrC. Warp ei dees Shay att OU Charles*hovbert μηδ, Βυ. ΘΟ Ἶ. JACKSONUs) Mor ea). gee en ee OrxiceRs*aNp Commirtenms FOR 1913-14" wh. i ee 3) en B28 List oF FELLOWS AND FormnIGN Honorary MEMBERS .. .. . 825 STATUTES ANDI STANDING ΘΟ Sy h lel geet) ) pale) --πππΠᾷ.ΠπΠ 990 IVUMEORDEEREMLUNIN SG oc) a) nt-n. teen eee el oe bie ea, Wadeeae se kom

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Proceedings of the American Academy of Arts and Sciences

Vou. XLVIII. No. 1.— May, 1912.

ON THE ULTRA VIOLET COMPONENT IN ARTIFICIAL LIGHT.

By Louis BELL.

WiTH Two PuaTEs

INVESTIGATIONS ON LIGHT AND Herat PUBLISHED WITH AID FROM THE

Rumrorp Funp.

THE ULTRA VIOLET COMPONENT IN ARTIFICIAL LIGHT.

By Louis BELL.

Presented March 13. Received March 25, 1912.

Purpose of the Investigation. —The fundamental purpose of this study has been definitely to evaluate the amount of energy given by various artificial illuminants in the ultra violet portion of the spectrum. In particular, beside determining the general proportion of ultra violet rays and their actual amount in each lamp investigated, the writer has determined in absolute measure the ultra violet energy delivered by each light source for unit illuminating value. Assuming that each of the artificial lights studied is to be used to produce a certain given illumination, the amount of. ultra violet radiation in- cidental to that illumination has been set down in absolute terms of ergs per second per sq. c. m. This classification of illuminants, which has not hitherto been made, is important in view of the possible harmful effects of radiation of short wave length which have been repeatedly discussed during the past few years. The amount of such possibly injurious radiation given by any particular lamp is a matter of small importance except as it is correlated with the illuminating power of the lamp, so that one may know to what amount of possibly harmful radiations he is exposed in securing a required degree of ilumination.

Nature and extent of Radiations under Suspicion as harmful. There has been much discussion concerning the effects of radiations of different wave lengths upon the eye. Without going extensively into an examination of the literature, which is very scattered and extensive, or of the physiological facts, some of which the writer now has under careful investigation and which will be reported later, it is sufficient here to say that the investigators of this matter may be divided into somewhat divergent schools. All agree that the extreme ultra violet rays, those of wave length less than 300 uu, which are absorbed by the cornea and so do not penetrate to the inner parts of the eye, produce when in sufficient intensity more or less serious damage to the corneal ephithelium, resulting in acute irritation, always accompanied by conjunctivitis, and sometimes by cloudiness of the cornea and other symptoms which go to make up the complex

2 PROCEEDINGS OF THE AMERICAN ACADEMY.

injury which has come to be known as ophthalmia electrica. It is in effect a superficial sunburn of the eye and is often accompanied by a similar sunburn in the vicinity of the affected eye. Whether this particular sort of effect is produced also by ultra violet rays of slightly greater wave length, say up to 320 μμ or 330 up, is a matter of some dispute, but most investigators have held this particular region under suspicion on account of the phenomena of snow blindness, which closely resemble those of the so-called ophthalmia electrica, and cannot be produced by the extreme ultra violet rays since the solar spectrum owing to atmospheric absorption is extremely weak at and below 300 mm, very near to which point it is wholly cut off. It is, however, fairly rich at 320 to 330 uy, the cutting off by atmospheric absorption being rather sudden, as shown in a, Plate 1.

Now while the cornea cuts off only rays of wave length less than 300 μμ the lens of the human eye ordinarily absorbs the whole ultra violet, it being substantially due to this absorption that we are unable to see beyond the violet. This absorption extends to about wave length 380 yu and in old persons in whom the lens gets slightly yellow even as far as wave length 420 μμ. In early youth there is a very slight transmissibility of the lens in the region 315 to 330 up as shown by Hallauer.t Now potentially the rays which are absorbed by a medium may produce changes in it and the ultra violet rays up to and including the extreme violet have been reputed by various writers to produce a large variety of lesions, including retinal injury due to the rays which may filter through the lens. The list of reputed dangers is a very long one including erythropsia, color scotomata, cataract and other serious results. The situation from the point of view of the ophthalmologists who seem to be really in fear of ultra violet radia- tions is well summed up by Schanz and Stockhausen.? Other writers like Best and Voege? attach relatively little importance to the effect of the ultra violet as such and are inclined to attribute some of the phenomena to over-intense radiation of ordinary light or to causes not connected to radiation at all.

A third group, of which Birch-Hirschfeld® is a representative, holds an intermediate view. It should be noted that the permanent

1 Klin. Monatsbl. f. Augemheilk., Dee. 1909.

2 Ztschr. f. Augenheilk., May 1910.

3. Klin. Monatsbl. f. Augenheilk., May 1909.

4 Die Ultravioletten Strahlen der modernen kuenstlichen Lichtquellen und ihre augenbliche Gefahr fiir das Auge. Berl., 1910.

> Ztschr. f. Augenheilk., July 1908, and elsewhere.

BELL. —~ ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 3)

injuries ascribed to ultra violet rays, like cataract and retinal degen- eration, are charged to the radiations running even up to the visible spectrum, while the extreme ultra violet, absorbed by the cornea, produces only superficial lesions generally recovered in a few days.

From the standpoint of the present investigation it did not seem justifiable to attempt to pass without further investigation on the validity of any of the divergent views here noted, but to deal with the radiations of short wave length as a whole, including in the possibly injurious group all those radiations which have been under serious suspicion on clinical evidence by reputable investigators. The line has therefore been drawn between the ordinary lighting radiations and radiations of short wave length in the extreme violet and ultra violet of the spectrum, where the lighting value of the rays is negli- gible and their actinic value notably high.

Separation of the Ultra Violet from the Visible Spectrum. Having determined on such a separation of the radiations under grave sus- picion of injurious action from the rest of the spectrum, it was next in order to find a suitable screen for making just this division of the spectrum, so that it would be possible to measure the energy in the two portions of the spectrum directly and as a whole, without a resort to the extremely difficult and troublesome measures of the energy in separate spectrum lines, a task of great delicacy when discontinuous have to be compared with continuous spectra. After considerable investigation a suitable medium was found in the so- called Euphos glass. This glass, which has been strongly recom- mended by Schanz and Stockhausen as eliminating completely all the harmful ‘rays and which was prepared under the direction of one of them, cuts off the ultra violet spectrum with remarkable definiteness while showing relatively little absorption of the general luminous rays.

Plate 1, b, c, d, shows the nature of this absorption very clearly. Spectrogram of this Plate is the spectrum of the mercury quartz arc put on merely for reference, the group at 365 wu being at the right of the figure and the brilliant green line exactly in the centre of the plate. Spectrogram c shows the spectrum of the magnetite are which is very rich in the ultra violet and d shows the same as absorbed by a Euphos glass screen 2 mm. thick. The exposure in each case was one minute with a rather wide slit and a very brilliant grating. The cut off of the shorter wave lengths by the Euphos glass in the ultra violet is very clean and sudden at wave length 390 uu, practically just at the end of the visible spectrum as seen by the average eye. The

4 PROCEEDINGS OF THE AMERICAN ACADEMY.

absorption continues slightly on into the violet, gradually fading away until the transmission becomes nearly complete for the bright blue mercury line (4385 pu).

In examining b, c and d of Plate 1 it must be remembered that the second order ultra violet overlaps the first order so that the group near 365 wu appears in the first order at the extreme right of the figure and in the second order at the extreme left. In d of this Plate the arc spectrum fades off on the left, not from absorption but from the weakening of the photographic action. The Euphos glass is ex- tremely transparent to the radiations throughout all except the ex- treme violet of the visible spectrum, and well into the infra red, as will hereafter be seen. The results here obtained for its absorption of the ultra violet are altogether parallel with those shown in the paper by Schanz and Stockhausen ® and also by Hallauer.?. The Euphos glass thus enables a particularly clean partition of the visible spectrum from the ultra violet and extreme violet to be made.

If it were possible to obtain an equally good absorbent for separat- ing the infra red from the visible spectrum radiometric measurements of efficiency would be greatly facilitated. It should here be noted that Euphos glass appears in various shades and some imitations of it are now upon the market, so that a sample of such glass should be tested in the spectrograph before use for such a purpose as the present, inasmuch as in some of the shades the cut-off of the ultra violet is much less sharp and complete. The sample here used was the original No. 1, 2 mm. thick.

Method of Investigation. —'The method taken for the evaluation was the familiar one of measuring the radiation directly by means of a thermopile connected with a sensitive galvanometer in a manner familiar in recent experiments on the efficiency of illuminants in the visible spectrum, 6. g., Lux,® Féry.2 The thermopile was chosen as the radiometric instrument merely as a matter of convenience. The instrument actually used was a Rubens linear thermopile, having 20 constantin-iron couples with a total resistance of 4.6 ohms. It was mounted as shown in Figure 1, in a vacuum tube with a quartz window immediately in front of the couples. The inner body of the instru- ment, containing the couples, was taken out of its original mounting and set up in a tube about 37 mm. in diameter, through the upper end of which was sealed a pair of leading-in wires.

6 Zts. f. Augenheilk., May 1910, Table VII, figure 3. 7 Archiv. of Ophthal., Jan. 1910, Plate I, figure 3.

8. Zts. f. Beleuchtungswesen, Heft 16, 1 p. 36, 1907. ® Bull. Soc. Franc. de Physique, p. 148, 1908.

BELL. ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 5

These were firmly clamped in the binding posts of the instrument by working through the side tube attached for the reception of the quartz window. The thermopile was then pushed up exactly opposite the side tube and wedged in place with cork and cotton wool attached with shellac. The end of the side tube was flanged out and ground flat for the fitting of the quartz window and after the shellac had dried out thoroughly the window was fastened in place and the lower end of the tube drawn out for the attachment of the pump. The tube was pumped to the high vacuum usual in an X-ray tube, and was then sealed. It was mounted as shown in a block of wood to which was secured the disconnecting terminal, reached by a long handled plug,

Figure 1. Vacuum thermopile. Figure 2. Quartz cell.

and the whole was then surrounded by a pasteboard case having a hole just opposite the quartz window, and packed full with loose cotton wool. The galvanometer was of the D’Arsonval type, having a sensibility of 210° ampere per mm. scale deflection. Its period for the attainment of a complete deflection, was, under the ordinary conditions of its use, 1 minute.

The galvanometer deflections were read by a scale and telescope, the scale being a special one bent to 1.5 meters radius. The thermo- pile indications were calibrated in absolute measure by observations

6 PROCEEDINGS OF THE AMERICAN ACADEMY.

on the radiation of a standard incandescent lamp supplied by the Bureau of Standards. After applying the proper correction for stray thermal losses and spherical reduction factor and reducing the read- ings as taken to the standard distance of 50 cm. employed throughout this investigation, the constant of the thermopile galvanometer system was found to be 1 mm. = 1 scale division = 35.3 ergs per second per square em. By this constant the observed deviations were reduced to absolute dynamical measure.

As a matter of convenience and to establish an approximate ratio between the ultra violet radiation from the various sources studied and the radiation in the visible spectrum, an absorption cell which

8

Transmission i

500 700 900 1100 1300 1500 μμ

Figure 3. Absorption curve of water.

eliminated nearly all the infra red was kept in front of the thermopile window. This cell, Figure 2, was of glass, ground flat and exactly 1 em. thick, 44 mm. external diameter and 35 mm. internal diameter. The glass ring was provided with a hole for filling and was closed by two quartz plates cut across the axis, each 2.25 mm. thick and 44 mm. diameter. These were fastened with hard shellac to the glass cell, and the cell in use was filled with distilled water. The absorption of a layer of distilled water of this thickness is shown in Figure 3 taken from Nichols’s experiments.’ Quartz has no material absorption in the part of the infra red spectrum transmitted and neither quartz nor

10 Nichols, Physical Review, Vol. 1, p. 1.

BELL. ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 7

distilled water in this thickness has any material absorption in even the extreme ultra violet up to the limit investigated.

The use of this cell therefore could produce no sensible effect on the accuracy of the ultra violet measurements, while it did serve the extremely useful purpose of limiting the total amount of energy to be measured and of eliminating any difficulties that might arise owing to absorption in the further part of the infra red, all the absorbing media incidentally used being, as compared with water, practically entirely transparent to all the radiations that got through the water cell. It would have been convenient if some substance cutting off the infra red sharply at 750 uu or 800 μμ had been available. Unfortu- nately, there is no such substance, so far as has yet been discovered, the very few substances less transparent than water in the region 800 to 1300 μμ being useless for the purpose of this investigation on account of opacity in the ultra violet and generally in the visible spectrum as well. Iron ammonium alum used by Lux (loc. cit.) and the copper salts used by Féry (loc. cit.) are open to this objection and the same is true of all the otherwise useful and promising sub- stances discussed in the very thorough and valuable researches of Coblentz."

In some of the experiments a second similar quartz cell was used, particularly in work on are lamps. In this case the Euphos glass used to cut out the ultra violet portion of the spectrum was perma- nently affixed to one of these cells and either the plain quartz cell or the Euphos-quartz cell was thrust into the beam so as quickly to get differential readings. In order to avoid the somewhat large correc- tion due to reflection of energy which would have been produced by the introduction of a plain slip of Euphos glass to cut out the ultra violet the following expedient was adopted.

The Euphos glass was attached to the surface of the quartz cell by spring clips with the addition of a thin capillary film of pure glycerine between the quartz and glass surface. Glycerine is immensely trans- parent to all radiations, including the extreme ultra violet, to which Canada balsam and gelatine are quite opaque. Its index of refrac- tion, 1.47 for D, is sufficiently near that for quartz and the various glasses to reduce the loss of light at the surfaces to an entirely negligi- ble amount. As the Euphos has a slightly less index of refraction than quartz, there was a minute residual gain in the total transmis- sion of the system when the Euphos glass was added, in the right direction to compensate for the minute losses by absorption in the glycerine film.

1 Bull. Bureau of Standards, Vol. 2, p. 619.

8 PROCEEDINGS OF THE AMERICAN ACADEMY.

As a check on the possible magnitude of this virtual absorption by the glycerine film readings were taken on a tungsten lamp through the quartz cell alone, and through the quartz cell plus a disc of optical crown glass 2 mm. thick secured with glycerine in the ordinary man- ner. The absorption of this crown glass is shown in Plate 1, e, f, g, in which e is the spectrogram of the quartz arc taken with a wide slit and 2 minutes exposure, f the spectrogram through the crown glass in question, and g through the Euphos glass. In spite of the fact that there is a slight absorption by the crown glass in the region near 300 up, the addition of the crown glass and glycerine film reduced the galvanometer deflection by barely 0.5 %, an amount scarcely out- side the errors of observation. ‘The energy cut off from the spectrum of a tungsten lamp by the crown glass would be of course very _ small, but perhaps not negligible, since as Schanz and Stockhausen have shown (loc. cit. table VIII, figure 6) the tungsten lamp spectrum goes quite down to 300 μμ in sufficient strength to give a clear photo- graphic effect. At all events it is evident that the use of the glycerine film involves no material errors.

In the ordinary experimentation in using steady sources, sets of readings were taken alternately with and without the Euphos glass, the glass being either added to the clear cell with the glycerine film, or removed and the film quickly washed away with distilled water. With sources which give trouble from unsteadiness the second quartz cell was brought into play as previously mentioned. Aside from a slight drifting of the zero point, which is generally observable in measurements with a thermopile, the method adopted worked very smoothly. The drift, however, was usually small and slow and satis- factorily taken care of by a time correction. With proper attention to this, the readings, although necessarily slow, were nearly as consis- tent as would be found in ordinary photometric measurements. The following string of deflections forming a single group of 5 readings is typical of those obtained under ordinary conditions.

Scale readings from bare quartz lamp through quartz cell only.

cm.

36.17 36.10 36 .27 36 .36 36 .16 Av. = 36.21

BELL. ——~ ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 9

The mean departure of a single reading from the average here given is slightly less than 4%, so that the errors of observation, of which this is a fair sample, showed that the thermopile observations are about as reliable as those with a photometer. Some preliminary experiments made on Euphos and other glasses showed that the transmission of the Euphoa glass aside from its absorption in the violet and ultra violet was exceptionally high for such rays as got through the layer of distilled water. In fact the total transmission of energy with Euphos glass was greater than with the ordinary samples of clear glass and was only exceeded by a single sample of optical crown which showed extraordinary transparency to all these radiations, so great that the losses were practically only those charge- able to actual reflection at the surfaces.

Measurements on various Illwminants. With these preliminaries the apparatus was set up permanently and work begun on commercial illuminants. Readings of current and voltage on the electric lamps were taken by Weston instruments freshly calibrated, and the quantity readings on the gas lamps tested were obtained from a newly adjusted standard meter.

100 Watt Tungsten The first source of light investigated was an ordinary 100 watt tungsten lamp, taking actually .951 amperes at 113 volts, i. e. 103.38 watts, and giving 79.4 ο. p. in the direction of the thermopile. With this lamp the mean difference of deflection due to energy cut off by the Euphos glass was 1.9 em. The ultra violet energy cut off, including such losses in the extreme violet as are indi- cated by Plate 1,d, was 6% of the total energy transmitted by the quartz cell.

100 Watt Gem. The second source studied was anordinary 100 watt Gem lamp, taking 100 watts at 114 volts and giving in the marked direction 39.25 c. p. This lamp of course gave a spectrum relatively weak in the ultra violet, but as will be seen from its spectrogram in Plate 2,6, the ultra violet region down to wave length 330 μμ is by no means negligible. The total differential deflection due to the ultra violet was in this case only 0.61 em., 2.6% of the total deflection. These readings confirm the extraordinarily small absorption of Euphos glass throughout the longer wave lengths, since the transmission ob- served with the known cut off of a very perceptible amount in the ultra violet, leaves no room for any material selective or general absorption elsewhere.

It should here be noted that while quartz transmits with extraordi- nary freedom, so far as absorption is concerned, all rays which are

10 PROCEEDINGS OF THE AMERICAN ACADEMY.

allowed to pass by a cm. thickness of distilled water, it still exercises a slight selective action by reflection. The index of refraction of quartz for the longer wave lengths of the visible spectrum is 1.54, while for rays in the further ultra violet this figure rises to about 1.6, hence in accordance with Fresnel’s formula (2>;) 2 there is a small amount of selective stopping of the ultra violet rays by reflection. This occurs both at the quartz water cell and at the quartz window in front of the thermopile so that the total selective effect is proportional to the fourth power of the difference due to the change in the index of re- fraction for a single surface of transmission. This difference amounts to approximately 2% as between the red rays and the further part of the ultra violet. The result is to cause a slight under estimation of the ultra violet. No account has been taken in any of these experi- ments of this very small and troublesome correction, which amounts in ordinary cases to only a small fraction of 1% of the total ultra violet. The existence of the effect should, however, be noted as it has a tendency toward causing a slight under estimate rather than an over estimate of the ultra violet component.

Cooper Hewitt Tube.—'The next source investigated was the Cooper Hewitt tube. One of the ordinary commercial 22 inch tubes was used, the particular tube having previously been used in another research and very carefully photometered. A section of this tube, giving 100 c. p., was screened off so that the length might be so re- duced that the energy from the whole,section taken could fall freely upon the thermopile without causing a material angular error or forcing one to depart widely from the standard distance of 0.5 meter. The horizontal radiation normal to the tube was of course taken, the reflector being removed. The corrected deflection due to the ultra violet amounted to 1.64 em. which corresponded to 41.7 % of the total energy passing through the quartz cell. The lamp was singularly steady and easy to work with, with the exception of producing an inconveniently small total deflection. The result, however, can be regarded as fairly precise in spite of the small magnitude, the mean deviation of a single reading amounting to barely over .5% in the total deflection. In this lamp the ultra violet energy is nearly all between 365 μμ and the visible spectrum, the extreme ultra violet being entirely cut off by the glass of the tube and the few lines of wave length between 365 and 300 μμ being reduced by the absorption to very feeble intensity. The total deflection produced by this lamp, of which the portion exposed radiated 100 ec. p., was only 17 % of the deflection given by the Gem lamp of the previous experiment, which gave less than 40 ec. p.

BELL. ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. ΠῚ

Quartz Mercury Lamp. Following the examination of the ordi- nary glass Copper Hewitt tube, the next source investigated was the quartz mercury lamp. Two tubes were available, each of the ordi- nary commercial 220 volt type rated at 3.5 amperes. One of these tubes, which is here referred to as the old mercury lamp, was made by the French Cooper-Hewitt Company and_had been already used for experimental purposes for about a year and had seen rather hard service, having often been worked above its rated amperage. The second lamp was entirely new, made in the Cooper-Hewitt factory in this country and was not at any time worked above its rating. The spectrum of the quartz lamp is extremely rich in certain portions of the ultra violet, particularly in rays of wave length less than 300 μμ. It is well shown in Spectrum e of Plate 1. The brilliant lines in this spectrum, counting from the violet, have wave lengths as follows:

4077 .84 2967 .27 4046 .55 2925 .38 3983 .96 2893 .60 3906 .47 2752 .80 3663 .27 2698 .88 3662.88 | 2655 .14 | 3654 .83 2653 .70 } 3650 .14 2652 .07 | 3341 .48 2536 .52 3131 84] 2483 87 3131.56 } 2482 76 3125 .67 2482.07 3027 .49 2399 .81 3025.61 | 2399 .43 3023 .43 2378 .39 3021 .50 2302 .65

The wave lengths here are taken at the value assigned by Stiles in A. u. It willbe observed that a number of the lines are associated in close groups which with small dispersion mass into heavy lines. The relative intensity of the lines, as is well known, shifts consid- erably with the degree of excitation of the tube, so that the relative intensities given by Stiles do not agree with the spectrograms taken from the quartz arc for the same reason that Stiles’ arc and spark intensities do not agree. The quartz arc spectrum resembles Stiles’ are spectrum much more closely than it does the spark spectrum.

12 Astrophysical Journ., Vol. XXX, p. 48.

12 PROCEEDINGS OF THE AMERICAN ACADEMY.

In particular the quartz are spectrum displays a very striking gap between wave length 334.14 μμ and the double line at wave length 313.1 up. Save for the very faint haze of continuous spectrum that characterizes the radiation from the quartz tube this part of the spectrum is blank. Indeed the line 334.14 uy itself is far from strong relatively to those in the further part of the ultra violet and there is. very little effect of radiation between wave length 313.1 μμ and 365.2 μμ. This gap is of some significance in interpreting the results: of bactericidal experiments, since any failure of bactericidal action in the region between wave length 350 wu and wave length 313 μμ observed in working with the quartz lamp may be due to the absence of any strong radiation in this region as well as to lack of specific bactericidal power in rays of this particular wave length if they existed.

In the radiometric investigations on the old quartz lamp it was run at 3.7 amperes and about 80 volts, an average of about 260 watts, without an external globe. Under these circumstances the corrected deflection due to the total ultra violet was 16.7 cm. The deflections: were not quite so steady as in the case of the ordinary Cooper Hewitt tube, but still the average departure of a single reading was within 1%. After the deflection due to the total ultra violet was determined another set of readings was taken with the bare lamp and quartz cell and then with the Euphos glass replaced by the crown glass: of which the absorption spectrum is shown at f, Plate 1.

This glass in effect cuts off substantially the whole of the extreme ultra violet spectrum, letting pass in practically undiminished strength only the lines of greater wave length than 300 wu. This separation is of some importance with respect to the bactericidal power of the lamp in water purification and similar work. The result was to show that the transmission of the crown glass was 54.7 % of the transmission found for the Euphos glass. In other words, nearly one half of the total ultra violet energy in this lamp was of wave length below 300 yu. Of the remaining half the spectrum shows, as just indicated, that by all odds the larger part lies between 365 μμ and the visible spectrum.

The new quartz lamp without its globe was then tested, the input in this case being 350 watts. The ultra violet output was greater than in the old tube, the total deflection reduced to the standard distance rising to 32.1 cm. In this case 65.1 % of the energy trans- mitted by the quartz water cell was cut off by the Euphos glass. Following up the radiometric measurement further, the Euphos glass was replaced by the light crown glass as before with the result of showing that substantially one half, 49.9 %, of the total ultra violet

BELL. ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. i153

was cut off by the crown glass and hence substantially this proportion was of wave length less than 300 wu.

In running quartz lamps without their globes, as was done in these experiments, the energy output is considerably diminished by the cooling of the tube and the light-giving properties of the lamp are very much reduced. Both the old and the new quartz lamps herein noted were photometered. The lamps were compared against a tungsten secondary standard by means of a Simmance-Abady flicker photo- meter. Thee. p. normal to the length of the tube and in a horizontal direction, was for the old quartz lamp 415, for the new quartz lamp 348, in each case without any enclosing globe. Both lamps were very steady and easy to work with, both on the photometer bar and with the thermopile.

Finally the new quartz lamp was fitted with its regular diffusing globe and tested with the thermopile. In working with the globe the tube operated at a higher temperature and far more intensively, the wattage rising to 400. With the Euphos glass in, the total change in deflection amounted to only 3.7 em. although the lamp tested on the photometer as in the previous case reached 820 c. p. in the hori- zontal direction. In percentage the amount of energy cut off by the Euphos glass was 42.5. These figures plainly indicate that the globe absorbed the further ultra violet very strongly, more strongly than the crown glass already referred to. In fact the deflection due to the ultra violet energy which passed through the globe of the lamp was extraordinarily small with respect to the ec. p. of the source, very much smaller than in the case of any other illuminant investigated. With- out the globe the quartz are is a very powerful source of radiation in the extreme ultra violet, below wave length 300 uu. With its ordinary globe on, all this energy in the extreme ultra violet is cut off and the small remaining amount, mostly in that part of the ultra violet nearest the visible spectrum, becomes quite insignificant.

The Welsbach Mantle— At this point study of the radiation from the Welsbach light was taken up. The particular form used was a Graetzin street lamp with a single large inverted mantle fitted with a clear glass globe, which obviously eliminated whatever of extreme ultra violet might be present. This burner took 6.4 feet of gas per hour at 3 inches pressure and gave 75 c. p. in the horizontal direction. Its total deflection was slightly greater than that produced by the quartz lamp with its globe tested immediately before. The addition of the Euphos glass cut down the deflection by .924 c. m., an amount equiva- lent to the absorption of 8.4 % of the total radiation recorded. The

14 PROCEEDINGS OF THE AMERICAN ACADEMY.

lamp proved fairly easy to work with in point of steadiness and the average variation of a single deflection from the mean was still less than 1%.

Acetylene Flame. Following the trial of the Graetzin lamp a series of measurements was made on an acetylene flame fed from a Prestolite tank. This flame gave on the photometer in the direction of measurement 27.35 c. p. and its change in deflection on interposi- tion of the Euphos was .524 cm., corresponding to a cut off of 4.5% of the total energy. It proved very amenable to measurements and was quite as steady and easy to work with as the mantle burner pre- viously used. The spectrum of the acetylene flame reaches well down into the ultra violet as shown by Schanz and Stockhausen." It reaches, in fact, approximately wave length 310 wu, but the further portion of the spectrum is comparatively weak. The spectrum of the Welsbach mantle with a clear globe, given by the same authorities (loc. cit.), is cut off at about wave length 320 μμ, but is notably bright in the part of the ultra violet toward the visible spectrum. These results are fully checked by the spectrograms taken of the particular burners here indicated.

The Carbon Electric Arc. Next in order the various are lamps were taken up for investigation, beginning with the are between carbon electrodes. On account of the relative instability of the ares the method of experimentation was modified. A second quartz cell similar to the one already in use was constructed and filled with distilled water. The ratio of the absorption between this new cell and the old cell was then determined. From a slight difference in thickness or in polish of the quartz plates the new cell was found to give about 1% more absorption than the original quartz cell and a correction for this difference was introduced in the subsequent meas- urements. The two quartz cells were mounted in recesses in a sliding screen so that either could be brought quickly in front of the thermo- pile window. The Euphos glass screen was then mounted with a glycerine film on one of the quartz cells so that the cells with and without the Euphos could be rapidly interchanged in the beam from the lamp under test and the absorption thus determined without having to depend on the constancy of the lamp for any considerable time.

The times of observation were regulated by means of a stop watch so that a time correction for shift of zero could be readily made, and

13 Zts. f. Augenheilk., V. XX XIII, plate 8.

BELL. ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 15

by taking several preliminary swings, so as to give the thermopile chance to settle into a steady state, the rate of shift of zero was kept pretty steadily and the corrections were easily applied. It was also necessary to photometer the ares in the actual condition in which they were under test. To this end the apparatus was set up as shown in Figure 4. Here A is the are lamp, B the thermopile, C the galva- nometer, D the telescope and scale, EK an adjustable rotating sector dise just in front of the are, F the quartz cells in their sliding screen in front of the thermopile window, G a silvered plate glass mirror which could be quickly interposed in the beam between the arc and

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waz A”

D

\ ve Tl pores es H ΘΟ ΖΜ Φ oe eat

Figure 4. Arrangement of radiometric apparatus.

the thermopile so as to deflect the rays into the portable photometer H, set up on the other side of the photometer room. The coefficient of reflection of the mirror had previously been many times determined as the mirror had been in use for photometric work. The photometer was ready for use at any time simply by closing the switch on the standard lamp. When in course of a series of thermopile measure- ments it was desired to test the ec. p. of the lamp the disc was started, the mirror swung into place and readings were then taken on the portable photometer.

16 PROCEEDINGS OF THE AMERICAN ACADEMY.

The carbon are was first attacked and it proved to be a difficult subject for investigation. The particular lamp used was of the en- closed type, having the globe fitted with a short side tube and a quartz window so as to keep the arc as steady as possible without losing the ultra violet. To the same end it was found desirable to adjust a magnet behind the are so as to keep it burning on the side of the carbons next the thermopile instead of wandering round and round the carbons in the usual manner.

The are thus operated gave a prodigious amount of ultra violet radiation, showing a continuous spectrum far down into the ultra violet and the three enormously intensive carbon bands usually ascribed to cyanogen, one of them in the extreme violet and the other two near wave lengths 380 yu and 360 μμ respectively. Reduced to the standard distance the deflection due to the ultra violet cut off by the Euphos glass amounted to 74 em., being 30 % of the whole energy which passed through the quartz cell. It has, of course, been long known that the naked electric are gives off very powerful ultra violet radiations and its effect in the production of ophthalmia electrica has been known for more than half a century, but in this case the extent of the ultra violet activity was somewhat unexpected.

It was undoubtedly considerably enhanced by the intensive cyano- gen bands as regards that portion of the radiation lying near the visible