S. P. Langley — Energy and Vision. 



361 



to the value of this absolute energy for different wave-lengths 

 in the normal spectrum which the subjoined table refers. This 

 table, which gives the energy as derived from thermal experi- 

 ments, rests on many thousand observations, taken, however, all 

 with what is called a high sun, i. e., with a sun more than 30° 

 above the horizon. As the distribution of this energy varies 

 somewhat from day to day, and particularly in the violet and 

 beyond, we have supplemented it by a series of direct observa- 

 tions taken with the bolometer on April 6, 1888, using the same 

 glass prism employed in the photometric work described later. 

 As those observations show a fair accordance with the others, 

 it is not necessary to repeat them. 







T 



ABLE 



[. Normal 



Spectrum. 









A=0^35 



0^38 



0^-40 



0^-45 



0^-50 



n-3 



0^-55 



o^-eo 



0^-65 



O^'IO 



20-7 



u 

 -768 



Heat= 1-8 



3-1 



5-3 



11-9 



20"7 



219 



22-2 



21-4 



20-2 



What has just been given in table I refers to the distribution 

 of energy in terms of lamp-black absorption, i. e., as u /ieat." 

 We now proceed to attempt to find it in terms of retinal ab- 

 sorption, i. e., as "light." It is well known that color pho- 

 tometry offers peculiar difficulties. My own experience, after 

 a long employment of the Rumford photometer for comparing 

 the relative intensity of different colored lights, is most un- 

 favorable to it, and I have also tried the Bunsen photometer 

 with almost equally unsatisfactory results. I have also experi- 

 mented with the ingenious photometer described by Masson 

 (Ann. de Ch. et de Ph., ser. Ill, xiv, p. 129), in which a disk 

 of paper, marked with black and white sectors, is revolved with 

 such rapidity that it assumes a uniform tint when viewed by the 

 colored light in question, but when illuminated by the electric 

 flash displays the sectors again. It is evident that the reappear- 

 ance of the sectors under the flash will be conditioned by the 

 nature of the light which furnishes the steady illumination. 

 But though on trial this has seemed to yield better results than 

 the ordinary photometers, the method is of difficult application 

 in connection with the particular apparatus about to be de- 

 scribed. I have therefore, after considerable experiment, de- 

 cided in favor of what may seem, at first, to be a cruder 

 method, but which is, I believe, for the present purpose, pref- 

 erable to any of the foregoing ; I mean the determination of 

 the intensity of light necessary to read a table of logarithms 

 or to discern any arbitrary characters. 



