20 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a flicker balance was established. The current read on K and re- 

 ferred 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 particu- 

 larly 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 pro- 

 duced 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 re- 

 flection of the nickel mirror, or approximately 21 % after the correc- 

 tion 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 w^ell frozen out. 



The observed difl'erence 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 atmos- 

 pheric 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 



