Prof. S. P. Langley on Energy and Vision. 

 Table I. — Normal Spectrum. 



\= 



0/^-35 



0^-38 



0'^-40 



0'*-45 



O'^-SO 



Of^-SS 



0k60 



0'*-65 



0'^-70 



(^■■15 



Cf-768 



Heat= 



1-8 



3-7 



5-3 



11-9 



17 3 



20-7 



21-9 



22-2 



214 



20-7 



20-2 



What has just been given in Table I. refers to the distribu- 

 tion of energy in terms of lampblack absorption, i. e. as 

 " heat.^^ We now proceed to attempt to find it in terms of 

 retinal absorption, i. e. as " light. ''^ It is well-known that 

 colour photometry offers peculiar difficulties. My own ex- 

 perience, after a long employment of the Rumford photometer 

 for comparing the relative intensity of different coloured 

 lights, is most unfavourable to it, and I have also tried the 

 Bunsen photometer with almost equally unsatisfactory re- 

 sults. I have also experimented with the ingenious photo- 

 meter described by Masson {Ann. de Ch. et de Ph. s^r. 3, 

 t. xiv. p. 129), in which a disk of paper marked with black 

 and white sectors is revolved with such rapidity that it as- 

 sumes a uniform tint when viewed by the coloured light in 

 question, but when illuminated by the electric flash displays 

 the sectors again. It is evident that the reappearance 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 connexion with the particular apparatus about to be de- 

 scribed. I have therefore, after considerable experiment, 

 decided in favour of what may seem, at first, to be a cruder 

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

 preferable to any of the foregoing ; I mean the determination 

 of the intensity of light necessary to read a table of loga- 

 rithms or to discern any arbitrary characters. 



Description of the Apparatus. 



The measurements have all been made in a dark room from 

 which every source of outside light is excluded except that 

 which enters the slit of the spectroscope. 



The light from the siderostat mirror M (fig. 1) passes 

 through a small aperture in the north wall and falls on the 

 slit (5i) (which has doubly moving jaws, 34 millim. high, 

 set in these experiments at a standard distance of 0*1 millim.), 

 then on the great collimating-lens (l) of 755 centim. focus 

 (aperture 11"9 centim.), t^ being a paper tube to prevent the 



