374 8. P. Langley — Energy and Vision. 



The light curves of F. W. V. (curve c, fig. 4), and of E. M., 

 (curve b, fig. 4), have their maxima respectively near ^=0 /z, 52 



and ^=(f -58. 



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, and 

 we can obtain an approximate estimate of this amount of energy 

 as follows : 



Actinometric measurements, made during the progress of the 

 photometric observations, showed a solar radiation of 1*5 calories 

 per square centimeter per minute. Of this amount of heat the 

 slit (s,), being 3 cm, 4 high by cm '01 wide, received the fraction 

 0*034. The visible spectrum from A to H, included, according 

 to the bolometer measures, about 21 per cent of the total en- 

 ergy, the absorption of the lower infra-red by the great thick- 

 ness of glass in the prism being large. We estimate that nearly 

 20 per cent had been lost by reflection before the bolometer 

 was reached. The spectrum formed had a length of 86 mm from 

 A to H. The average energy which passed through the milli- 

 meter aperture of slit s % was therefore (within these limits and 

 expressed as heat), 



l cal '5 X 0-034 X 0-21 X 0-8 X — , 



86 



or approximately i-jj-j-oir calorie, let us say 4,000 ergs per 

 minute. 



At 1 meter from slit s 2 , this energy is further spread out over 

 an illuminated area of 28 sq. cm., of which the square centi- 

 meter of fine print, being placed at an angle of 45° with the 

 path of the ray, occupies only about ^-. If a length of l mm of 

 the standard spectrum receives an average energy of 10 } 00 

 calorie per minute, the actual working part of the screen, con- 

 sisting, of the little square of fiue print, will receive at a distance 

 of 1 meter 400 \ o 7 calorie per minute. But this by no means 

 gives the amount of energy requisite to produce vision, since the 

 eye is able to receive a distinct visual impression in less than 

 one-half second of time. We may say, therefore, that a lumi- 

 nous energy of T o \ 6 calorie is sufficient to give a distinct 

 view of the small square of figures in the 'brightest part of the 

 spectrum, even after the immense loss of light by absorption and 

 diffusion in the paper, which may amount to -|-| of the whole. 



Even less light is needed to give the bare impression of lumi- 

 nosity. The sensitiveness of the human eye is indeed so extra- 

 ordinary, that the chief difficulty in measuring its power is to 

 find means for sufficiently reducing the intensity of sunlight, 



