372 8. P. Langley — Energy and Vision. 



It will be observed that no correction has been introduced 

 for selective absorption in the substance of the prism itself, as 

 this is absolutely negligible within the limited range of the 

 spectrum we are discussing. 



This table exhibits the relative effect upon very different 

 eyes of a given amount of energy in the form of radiation of 

 various wave-lengths. 



Quite notable differences exist between the different ob- 

 servers, not only as to the absolute sensitiveness of the eye, but 

 also as to the relative efficiency for different colors. This seems 

 to be, to some extent, a function of the age of the observer, if 

 we may draw any conclusion from so few comparisons, the 

 younger eyes being much more sensitive to the rays of shorter 

 wave-length. Beyond this, any unusual efficiency for a partic- 

 ular part of the spectrum is perhaps apt to be balanced by a 

 deficiency in another part, which, if strongly pronounced, 

 would be termed color blindness. Prof. J. Clerk-Maxwell, 

 employing pure spectrum colors, formed white by combining 

 26*3 per cent of red with 30*2 per cent of green and 43*5 per 

 cent of blue (Phil. Trans. E. Soc, 1860, p. 79) and on another 

 occasion with a slightly different apparatus (loc. cit. p. 74) the 

 same observer made white by mingling 21*9 per cent of red 

 with 33*3 per cent of green and 44 -8 per cent of blue. The 

 Allegheny observers, F. W. V., B. E. L., and E. M., with whom 

 this experiment was repeated, required from one fourth to one- 

 tenth less red and one-sixth to one-eighth more blue than Max- 

 well, forming white by mingling 20 per cent of red with 30 

 per cent of green and 50 per cent of blue. Since, in order to 

 make white, more of that color is required for which the eye 

 is most sensitive, we may perhaps infer that Prof. Maxwell was 

 somewhat less sensitive to blue than these observers, although it 

 should be remembered that the relative intensity of the blue 

 and red in the solar spectrum is liable to undergo considerable 

 fluctuations, so that where direct comparison of individual eyes 

 is impossible, some uncertainty must remain. 



We have selected for comparison with our results the follow- 

 ing by Capt. Abney (using a different photometric method), 

 which we have here reduced to the normal scale. (See " Trans- 

 mission of Sunlight through the Earth's atmosphere," by Capt. 

 W. de W. Abney, K.E., F.P.S.; Phil. Trans. K. Soc, vol. 178, 

 (1887), A., pp.. '274-276). From the mean of the observa- 

 tions of July 1st, July 5th and July 2lst, 1886, made with an 

 average air-mass of 1*33 atmospheres, we obtain these photo- 

 metric values for the normal spectrum : 



0^-70 

 0-5 



^ = 0^40 



</-45 



0^-50 



(f-55 



0^-60 



0^65 



Lights 0-8 



2-8 



25-0 



82-0 



66-5 



12'3 



