BELL. — THE PHYSIOLOGICAL BASIS OF ILLUMINATION. 95 



the practical effect of chromatic aberration. From a distance of a 

 couple of meters sharp definition of the grid is quite impossible. The 

 purple chosen should give considerable absorption of the green, yellow, 

 and orange, leaving strong red and blue evenly balanced in luminosity, 

 and the background should be of not greatly different luminosity, so 

 that the eye must rely mainly upon color effects. The rays from the 

 grid are then of two widely different colors, for which the focal length of 

 the eye differs. There are therefore two image surfaces of about equal 

 intensity perhaps half a millimeter apart, and the effect is a curious 

 blur, the eye hunting in vain for something definite upon which to focus. 



Interposing now a deep red screen (concentrated saffronine is good), 

 or a suitable blue screen, the image of the grid becomes nearly mono- 

 chromatic and appears sharply defined. This is an extreme case, but 

 any monochromatic light has an advantage in definition if other con- 

 ditions are at all favorable. It seems highly probable that the well- 

 known trouble found at twilight in trjdng to work by a mixture of 

 natural and artificial light is due to a similar cause. The predominant 

 hue of diffused sky light is strongly blue, while that of gas flames, incan- 

 descent lamps, and like sources, is strongly yellowish. At a certain 

 point in the fading of daylight the luminosities of these widely different 

 colors should balance closely enough to produce something of the effect 

 just described, although the usual difference of direction in the two su- 

 perimposed illuminations may play a part in the general unpleasant 

 effect. 



There is, however, an inherent danger in using monochromatic or 

 strongly colored light for general purposes. Whatever may be the 

 nature of color vision, a strongly colored light utilizes only a part 

 of the visual apparatus. If of high intensity to make up for inherently 

 low luminosity, it rapidly exhausts that part, and produces, as is well 

 known, a temporary color blindness. There is at least a serious chance 

 that long continued use of colored light would produce persistent and 

 perhaps permanent damage to color perception. A light nearly white, 

 with its maximum luminosity near the normal wave-length, runs the 

 least chance of imposing abnormal strains on the visual apparatus. 



In color discrimination the same rule holds good, for any considerable 

 departure irom white leads to entirely false color- values. In closing I 

 may mention an interesting question which arises with reference to 

 obtaining a light of high efficiency by building it up irom the mono- 

 chromatic primary components. Would the eye see clearly by such a 

 light, and could it discriminate colors properly 1 The answer is prob- 

 ably yes. The equation for white is roughly 



W=.20B + .30G + .50B. 



