94 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Acuity in practical degree is chiefly an attribute of cone vision. The 

 general theory of optical resolution requires acuity inversely as the 

 wave-length of the light concerned. In practice this difference is in 

 great measure masked by other and larger causes of variation. Chief 

 among these is the very low luminosity of the shorter wave-lengths on 

 the one hand and of the very long ones on the other. For example, in 

 comparing acuity at A = 500 fi/j. and A = 650 yu/x there is a proportional 

 difference really due to color, but a ratio of 2.5 : 1 in luminosity in fur- 

 ther favor of the green. Violet light favors acuity, if one can get 

 enough of it, but a luminosity of .02 of the maximum in the yellow 

 stands in the way. 



Certain strongly colored lights, like the flaming calcium fluoride arc 

 and the mercury arc, give apparently extremely sharp definition in black 

 and white objects. In general this is not due to any advantage in color 

 as such, but to improvement in the conditions of chromatic aberration 

 in the eye. At rest for distant vision, the normal eye is in focus for the 

 rays of maximum luminosity, and the focus for blue lies perhaps 0.4 mm. 

 in front of the retina. That is, the eye is short-sighted for short rays. 

 In near vision the rear conjugate focus moves backwards and the eye 

 finds focus on the blue with less accommodation than usual. Thus 

 Dow 14 finds that, while the mercury arc gives easy and sharp definition 

 for near vision, at a distance of twenty feet or even less it becomes 

 difficult to get focus. Lord Rayleigh 15 noticed some years ago that in 

 very weak light he became myopic and required a glass of— 1 diopter to 

 restore normal vision. This effect is of the order of magnitude required 

 by the shift of maximum luminosity into the green at very low intensi- 

 ties. Another phase of chromatic aberration is even more important. 

 Were it not for the existence of a very high maximum in the luminosity 

 curve, distinct vision would be impossible, since the difference of focus 

 between the red and violet in the eye is something like 0.6 mm. ; and 

 were these extreme colors highly luminous, there would be no focal sur- 

 face to which the eye could adjust itself. Only the great predominance 

 of the central colors in luminosity gives the chance for a fairly sharp 

 image. 



It is easy to show the difficulties into which equal luminosity 

 throughout the spectrum would plunge us. If one forms a grid of cer- 

 tain purples by cutting strips of tissue paper of the required color per- 

 haps 5 mm. wide and 100 mm. long and pasting them upon a dark 

 neutral background spaced about their width apart, one readily finds 



14 The Illuminating Engineer, 2, 20 et seq. 

 » Nature, 31, 340. 



