10 PKOCEEDINGS 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 (^-^) ^ 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 eft'ect 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 oft' 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 cm. 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 /x^i 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 ixfj. being reduced by the absorption to 

 very feeble intensity. The total deflection produced by this lamp, 

 of which the portion exposed radiated 100 c. p., was only 17 % of the 

 deflection given by the Gem lamp of the previous experiment, which 

 gave less than 40 c. p. 



