734 HANDBOOK OF PHYSIOLOGY. 



three large triangles. The more numerous and varied the parts of which 

 a figure is composed the more scope does it afford for the play of the 

 attention. Hence it is that architectural ornaments have an enlivening 



Fig. 433 A. 



effect on the sense of vision, since they afford constantly fresh subject 

 for the action of the mind. 



Color Sensations. If a ray of sunlight be allowed to pass through 

 a prism, it is decomposed by its passage into rays of different colors, 

 which are called the colors of the spectrum ; they are red, orange, yellow, 

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

 their course by the prism, and the violet the most, while the other colors 

 occupy 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 ther- 

 mometer, 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, 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. 



(1.) 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 Avhich 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 different 

 degrees, the sensation of white being produced at the same time 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 are strongly stimulated, the resulting sensation 

 being 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 (fig. 434) . 



(2.) The second theory of color (Bering's) supposes that there are six 



