COLOE-VISION AND COLOR-BLINDNESS. 689 



of the solar spectrum, others absorb one only, others absorb portions of 

 one or more. Wliatever remains is transmitted through the media of 

 the eye, and in the great majority of the human race, suffices to excite 

 the retina to a characteristic kind of activity. Few things are more 

 curious than the multitude of different color sensations which may be 

 produced by the varying combinations of the three simple elements, 

 red, green, and violet ; but this is a part of the subject into which it 

 would be impossible for me now to enter, and with which most of those 

 who hear me must already be perfectly familiar. 



Apart from the ett'ect of color as one of the chief sources of beauty 

 in the world, it is manifest that the power of distinguishing it adds 

 greatly to the acuteuess of vision. Objects which diifer from their sur- 

 roundings by differences of color are far more conspicuous than I hose 

 which differ only by differences of light and shade. Flowers are 

 much indebted to their brilliant coloring for the visits of the insects by 

 which they are fertilized ; and creatures which are the prey of others 

 find their best protection in a resemblance to the colors of their envi- 

 ronment. It is probably a universal truth that the organs of color 

 perception are more highly specialized and that the sense of color is 

 more developed in all animals in precise proportion to the general 

 acuteuess of vision of each. 



From a variety of considerations, into which time will not allow me 

 to enter, it has been concluded that the sense of color is an endowment 

 of the retinal cones, and that the rods are sensitive only to differences 

 in the quantity of the incident light, without regard to its quality. 

 iSIocturnal mammals, such as mice, bats, and hedgehogs, have no cones ; 

 and cones are less developed in nccturnal birds than in diurnal ones. 

 Certain limitations of the human color sense may almost be inferred 

 from the anatomy of the retina. It is found, as that anatomy would 

 lead us to suppose, that complete color sense exists only in the retinal 

 center, or in and immediately around the yellow spot region, and that 

 it diminishes as we pass away from this center towards the periphery. 

 The precise facts are more difficult to ascertain than might be supposed ; 

 for although it is easy to bring colored objects from the circumference 

 to the center of the field of vision on the perimeter, it is by no means 

 easy to be quite sure of the point at which the true color of the ad- 

 vancing object can first be said to be distinctly seen. Much depends, 

 moreover, on the size of this advancing object, because the larger it 

 is the sooner will its image fall upon some of the more sparsely distrib- 

 uted cones of the peripheral portion of the retina. Testing the mat- 

 ter upon myself with colored cards of the size of a man's visiting card 

 I find that I am conscious of red or blue at about 40° from the fixing 

 point, but not of green until it comes within about 30° ; while, if I take 

 three spots, respectively of bright red, bright green, and bright blue, 

 each half a centimetre in diameter and separated from its neighbor on 

 either side by an interval of half a centimetre, spots which would be 

 H. Mis, 129 44 



