224 HAND-BOOK OF PHYSIOLOGY. 



blue, indigo, and violet. The red rays are the least turned out of their 

 course by the prism, and the violet the most, whilst the other colors oc- 

 cupy in order places between these two extremes. The differences in the 

 color of the rays, depend upon the number of vibrations producing each, 

 the red rays being the least rapid and the violet the most. In addition 

 to the colored rays of the spectrum, there are others which are invisible, 

 but which have definite properties, those to the left of the red, and less 

 refrangible, being the calorific rays which act upon the thermometer, and 

 those to the right of the violet which are called the actinic or chemical 

 rays, which have a powerful chemical action. The rays which can be 

 perceived by the brain as visual rays, i.e., the colored rays, must stimu- 

 late the retina in some special manner in order that colored vision may 

 result, and two chief explanations of the method of stimulation have been 

 suggested. The one, originated by Young and elaborated by Helmholtz, 

 holds that there are three primary colors, viz., red, green, and violet, and 

 that in the retina are contained rods or cones which answer to each of 

 these primary colors, whereas the innumerable intermediate shades of color 

 are produced by stimulation of the three primary color terminals in differ- 

 ent degrees; the sensation of white being produced when the three elements 

 are equally excited. Thus if the retina be stimulated by rays of certain 

 wave length, at the red end of the spectrum, the terminals of the other 

 colors, green and violet, are hardly stimulated at all, but the red terminals 

 being strongly stimulated, the resulting sensation is red. The orange 

 rays excite the red terminals considerably, the green rather more, and the 

 violet slightly, the resulting sensation being that of orange, and so on. 



The second theory of color (Hering's) supposes that there are six 

 primary color sensations, of three pair of antagonistic or complemental 

 colors, black and white, red and green, and yellow and blue, and that these 

 are produced by the changes either of disintegration or of assimilation 

 taking place in certain substances, somewhat it may be supposed of the 

 nature of the visual purple, which (the theory supposes to) exist in the 

 retina. Each of the substances corresponding to a pair of colors, being 

 capable of undergoing two changes, one of construction and the other of 

 disintegration, with the result of producing one or other color. For in- 

 stance, in the white-black substance, when disintegration is in excess of 

 construction or assimilation, the sensation is white, and when assimilation 

 is in excess of disintegration the reverse is the case; and similarly with 

 the red-green substance, and with the yellow-blue substance. When the 

 repair and disintegration are equal with the first substance, the visual 

 sensation is grey; but in the other pairs when this is the case, no sensa- 

 tion occurs. The rays of the spectrum to the left produce changes in the 

 red-green substance only, with a resulting sensation of red, whilst the 

 (orange) rays further to the right affect both the red-green and the yellow- 

 blue substances; blue rays cause constructive changes in the yellow-blue 



