courtesy of the Director, this part of the work was done in the Rogers Laboratory of Physics where the conditions for getting natural light. were good. In Figure 5, A is a porte lumiére receiving the light from the sun and forming by means of the iris diaphragm B, stopped to 3 mm. diameter, an image of the sun on the thermopile front at C, before which was placed the usual quartz cell D. The thermopile was connected with the galvanometer F, read by the telescope and scale G. By the use of the diaphragm, forming a species of "pin hole" image on the face of the thermopile, at a distance of 3 meters,

the light and energy were cut down so as to be readable with comparative ease.

To measure the intensity of the illumination a Simmance-Abady flicker photometer H was set up close alongside the thermopile so that the solar image could be quickly moved so as to fall squarely on the photometer disc. On the other side of the photometer at I was an 80 watt tantalum lamp which was previously calibrated, in terms of the current flowing through it, against a standardized Gem lamp. From the source of supply the current was taken to this lamp through an adjustable rheostat J and a mil-amperemeter K. In measuring the light-intensity of the beam which was allowed to fall on the thermopile, it was simply shifted from the face of the thermopile to the face of the photometer and by means of the rheostat J

a flicker balance was established. The current read on K and referred to the standardization curve at once gave the c. p. of I, so that the illumination could be computed.

The mirror at A was an electrolytic nickel surface highly polished, inasmuch as nickel gives a considerably higher coefficient of reflection near the end of the solar spectrum than does silver, which is particularly weak at this point. To separate the extreme violet and ultra violet as before and on exactly the same basis, the solar readings were taken with simply the quartz cell and then with the Euphos glass and a glycerine film. The cut off of violet and ultra violet produced by the Euphos glass in the first day's readings was 16.2% and in a second day's reading 17.9%, both days being brilliantly clear and cold in late December at noon. The average energy therefore cut off was substantially 17% uncorrected for the coefficient of reflection of the nickel mirror, or approximately 21% after the correction for the variation in reflection as between the ultra violet and the visible spectrum.

This figure is somewhat large as compared with the data ordinarily quoted for the ultra violet component of the solar spectrum, but it should be noted that this comparison is not with the spectrum as a whole but with that portion of it transmitted by a quartz cell filled with distilled water which cuts off a large part of the infra red. Also the absorption of the Euphos glass extends into the violet as has been previously noted, and finally the observations were taken in cold winter weather when the aqueous vapor, which is important in the absorption of the atmosphere, is pretty well frozen out.

The observed difference of deflection in these experiments on the sun due to the cut off of the ultra violet was 2.28 cm. and the observed intensity of the illumination was equivalent to 101 foot candles. These readings show precisely what the general theory indicates, that the solar light must be regarded as received from an enormously hot and hence very efficient radiator which has been robbed by atmospheric scattering and absorption of a considerable part of its shorter wave lengths.

RECORD OF GENERAL RESULTS.

In these experiments the following artificial sources of light were investigated with respect to the ultra violet component of each as separated from the rest of the spectrum by a disc 2 mm. thick of Euphos glass 1:-G. E. M. lamp; tungsten lamp; Cooper Hewitt

tube; quartz lamp of the French Cooper Hewitt Company without globe; quartz lamp, American, without globe; quartz lamp, American, with globe; Graetzin mantle burner; acetylene flame; carbon electric arc through quartz window; magnetite arc through quartz window; magnetite arc with ordinary globe; Nernst glower. In addition, a study was made of sunlight with the thermopile for comparative purposes and spectrographic studies were also made of the ordinary yellow flame arc and of the arc between iron terminals such as is used for therapeutic purposes. The Euphos glass was chosen as the medium for the partition of the ultra violet from the rest of the spectrum for the reason that it cuts out and was intended to cut out by its designers all the rays of any illuminant which are under indictment as having specific harmful action on the eyes.

Broadly, the accusations of short wave lengths as injurious to the eye involve the entire ultra violet from the furthest point reached by natural or artificial illuminants up to and into the chemically active rays of the violet. If on the one hand it is the rays in the extreme ultra violet, wave length 300 μμ and less, which are absorbed by the cornea, that are held responsible for the ordinary phenomena of ophthalmia electrica, it is the rays of ultra violet of greater wave length than this, extending clear into the violet, that have been regarded by some recent investigators as producing perhaps serious lesions of the retina and of the lens. Note Schanz and Stockhausen. 15 The former class of injuries which have to do with the radiations absorbed by the cornea are wholly superficial and, according to Van Lint 16 the prognosis is generally good and the recovery rapid. Injuries to the retina and the lens, in-so-far as they take place, involve a far greater danger of permanent injury. Glass-blowers cataract is one of the typical injuries which has been ascribed to ultra violet radiations lying adjacent to the visible spectrum by Schanz and Stockhausen, BirchHirschfeld and others. Obviously, the temperature of melting glass (1400° C) is too low to give rise to any material amount of energy in the extreme ultra violet.

The present investigation, therefore, took account of the whole body of radiations of short wave length. So far as possible injury from the ultra violet component in any artificial light source is concerned it is obviously dependent on the amount of actual energy delivered by the source in the ultra violet region and not upon the

15 Ztschr. f. Augenheilk., Mai, 1910.

16 Accidents oculaires provoqué's par l'électricité, p. 100.

percentage relation of this energy to the whole input. It is quite clear that in order to do any injury to the eye a certain amount of energy must be spent upon it and must be delivered at a rate in excess of the power of the eye to repair damages. One receives injury from excessive exposure to ultra violet rays just as he receives it by excessive exposure to heat rays. In either case the delivery of energy at a very high rate for a considerable time does damage.

At a moderate rate and for a moderate time the constructive forces of the organism are not over balanced by the destructive forces of the radiations. Hence the first application of the data obtained from the sources investigated was to determine the actual rate at which ultra violet energy was delivered by them. Table I shows for all the electric sources of light, of which the input could be readily measured, the gross input in watts at the lamp terminals, the total ultra violet radiation in ergs per second per square cm. at the standard distance of half a meter and finally, this ultra violet output in terms of ergs square cm. per second per watt input. This last column is proportional to the efficiency of the source as a producer of ultra violet radiations in terms of the gross input.

In Table I the highest ultra violet output per watt of input is reached by the carbon arc operated in the manner already described. The next highest figure is given by the quartz lamp operated without its globe, a condition of relatively low luminous efficiency which would only be found in cases where the arc was being used for bactericidal purposes or other special tasks where ultra violet radiations are

important. The very high ultra violet output reached by the carbon arc is as has already been pointed out largely due to the very intensive cyanogen bands about in the middle of the ultra violet spectrum and the output of wave length below 300 uu is materially less than it is in the quartz lamp operated without its globe.

At the other end of the list stand the G. E. M. lamp and the ordinary Cooper-Hewitt tube, the former showing a very low ultra violet output by reason of its relatively low temperature and the latter by reason of the fact that the extreme ultra violet is entirely cut off by the tube, and the middle ultra violet being very weak in the mercury spectrum, the main body of the energy is of wave length greater than 365 μμ. In fact since the spectrum of the G. E. M. lamp runs down nearly to wave length 300 μμ, and is strong only between say 360 and the visible, the energy distribution of the spectra of these two illuminants is singularly similar, considering their wide difference in character.

The Nernst and tungsten lamps produce rather more total ultra violet than the Cooper-Hewitt tube, most of the output being toward the visible spectrum. The Nernst lamp operated without its globe gives a spectrum relatively stronger in the further ultra violet, reaching wave length 300 μμ with a considerable degree of strength and stretching beyond it. All the lamps running with glass globes show a weak spectrum in that region. For this reason the quartz lamp with its regular diffusing globe shows an ultra violet output per watt almost as low as the G. E. M. lamp, the cut off of the globe in the ultra violet region being very striking. The magnetite are both with and without its globe gives a considerable ultra violet output. The globe cuts off much less ultra violet than in the case of the quartz lamp, the latter being relatively rich in the rays which the glass most effectively absorbs.

Table II shows the percentage of energy cut off by the Euphos glass in each of the illuminants investigated as compared with the total. energy which was transmitted by the quartz water cell, and also the relative horizontal c. p. of the sources dealt with. The percentage ratios of ultra violet are therefore numerically higher than they would be in the case of admitting the whole infra red to the thermopile. The relative composition of the various sources, however, is well expressed by the data.