THE SENSE OF VISION. 779 



Furthermore, the sensation produced by the impact of ether waves of a definite 

 length will vary according as the eye is simultaneously affected by a greater or 

 less amount of white light. This modification of the sensation is termed its 

 degree of " saturation/' light being said to be completely saturated when it is 

 " monochromatic" or produced by ether vibrations of a single wave-length. 



The modifications of light which taken together determine completely the 

 character of the sensation are, then, three in number viz. : 1. Color, depend- 

 ent upon rate of vibration or length of the ether wave ; 2. Intensity, dependent 

 upon the energy of the vibration ; 3. Saturation, dependent upon the amount 

 of white light mingled with the monochromatic light. These three qualitative 

 modifications of light must now be considered in detail. 



Color. In our profound ignorance of the nature of the process by which, 

 in the rods and cones, the movements of the ether waves are converted into a 

 stimulus for the optic nerve-fibres, all that can be reasonably demanded of a 

 color theory is that it shall present a logically consistent hypothesis to account 

 for the sensations actually produced by the impact of ether waves of varying 

 rates, either singly or combined, upon different parts of the retina. Some of 

 the important phenomena of color sensation of which every color theory must 

 take account may be enumerated as follows : 



1. Luminosity is more readily recognized than color. This is shown by 

 the fact that a colored object appears colorless when it is too feebly illuminated, 

 and that a spectrum produced by a very feeble light shows variations of inten- 

 sity with a maximum nearer than normal to the blue end, but no gradations 

 of color. A similar lack of color is noticed when a colored object is observed 

 for too short a time or when it is of insufficient size. In all these respects the 

 various colors present important individual differences which will be considered 

 later, 



2. Colored objects seen with increasing intensity of illumination appear 

 more and more colorless, and finally present the appearance of pure white. 

 Yellow passes into white more readily than the other colors. 



3. The power of the retina to distinguish colors diminishes from the centre 

 toward the periphery, the various colors, in this respect also, differing mate- 

 rially from each other. Sensibility to red is lost at a short distance from the 

 macula lutea, while the sensation of blue is lost only on the extreme lateral 

 portions of the retina. The relation of this phenomenon to the distribution 

 of the rods and cones in the retina will be considered in connection with the 

 perception of the intensity of light. 



Color-mixture. Since the various spectral colors are produced by the dis- 

 persion of the white light of the sun, it is evident that white light may be 

 reproduced by the reunion of the rays corresponding to the different colors, and 

 it is accordingly found that if the colored rays emerging from a prism, as in 

 Fig. 237, are reunited by suitable refracting surfaces, a spot of white light will be 

 produced similar to that which would have been caused by the original beam 

 of sunlight. But white light may be produced not only by the union of all 

 the spectral colors, but by the union of certain selected colors in twos, threes, 



