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



risons, the younger eyes being much more sensitive to the 

 rays of shorter wave-length. Beyond this, any unsual effi- 

 ciency for a particular part of the spectrum is, perhaps, apt 

 to be balanced by a deficiency in another part, which, if 

 strongly pronounced, would be termed colour-blindness. Prof. 

 J. Clerk- Maxwell, employing pure spectrum-colours, 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. R. Soc. 

 1860, p. 79) ; and on another occasion, with a slightly different 

 apparatus (Joe. 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 P. W. 

 v., B. E. L., and E. M., with whom this experiment wa? 

 repeated, required from one fourth to one tenth less red and 

 one sixth to one eighth more blue than Maxwell, 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 colour is required for which the eye is 

 most sensitive, w^e 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 fol- 

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

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

 " Transmission of Sunlight through the Earth's Atmosphere," 

 by Capt. W. de W. Abney, R.E., F.R.S., Phil. Trans. R. Soc. 

 vol. clxxviii. (1887), A., pp. 274-276.) From the mean of 

 the observations of July 1st, July 5th, and July 21st, 1886, 

 made with an average air-mass of 1*33 atmospheres, we obtain 

 these photometric values for the normal spectrum : — 



X= 0''-40 0'''45 O'^-SO C'-SS 0'"-60 Q^-Q5 Qt'^-IQ 

 Light= 0-8 2-8 25-0 82-0 66-5 12-3 0-5 



The general form of this curve agrees with that of S. P. L. 

 (curve a, fig. 4), showing a maximum sensitiveness near \= 

 0'^-57. The light-curves of F. W. V. (curve c, fig. 4) and of 

 E. M. (curve 5, fig. 4) have their maxima respectively near 

 X=0''-52 and X= 0^-53. 



Everything which has preceded has had reference to the 

 relative luminous effects produced by any (moderate) constant 

 quantity of energy. It may, however, be interesting to make 

 the novel calculation as to the actual amount of energy either 

 in horse-power or any other unit, required to make us see, 



