BELL. — ULTRAVIOLET COMPONENT IN ARTIFICIAL LIGHT. 9 



The mean departure of a single reading from the average here given 

 is shghtly less than 1%, 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 Illuminants. — 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. 103.3 watts, and giving 79.4 c. 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 cm. 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 an ordinary 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, b, the ultra violet region down to wave length 330 ^c^t is by no 

 means negligible. The total differential deflection due to the ultra 

 violet was in this case only 0.61 cm., 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 



