THE PHENOMENA OF EXCITATION 537 



red because it permits more of the red rays than of any other kind to pass 

 through it. We must remember, however, that it is only the relative number 

 of rays reflected or transmitted which gives the color to a nonluminous object ; 

 for rays of other colors may as a rule be reflected or transmitted at the same 

 time. 



If all the light rays which fall upon a surface are reflected in the same 

 relative numbers as they occur in colorless light the surface appears white, 

 gray or black according as the total quantity of rays reflected is great or 

 small. The same holds mutatis mutandis for transparent objects. 



Whether a surface is colored or not its brightness depends upon the quan- 

 tity of reflected rays: a bright red surface reflects much light of which a rela- 

 tively large number of the rays are red ; a dark red surface reflects the same 

 rays in relatively largest number, but the total quantity of light reflected by 

 it is relatively small. 



Two inferences which may be drawn from the facts presented here should 

 be kept steadily in view throughout the following discussion : that white light 

 always consists of rays of different wave lengths 

 and that the only really pure colors are the colors 

 of a pure spectrum. 



2. THE PHENOMENA OF EXCITATION 



When light falls upon a freshly exposed retina, 

 the latter undergoes a number of changes which 



are objectively demonstrable. Thus a pigment, FIG. 231. Optogram formed 



present in all parts of the retina except the yellow on the retina b ~ v action of 



-, nni j> -.L i 11 T light on the visual purple. 



spot and called because of its color the visual pur- a f ter Kuhne The purp ie 

 pie, fades (Fig. 231) ; the coloring matter of the fades and the image thus 

 pigment epithelium (frog's eye, Fig. 232) moves formed is fixed in a solution 

 inward, being accompanied in this movement by ^ 

 a shifting of the cone cells in the same direction, graphic plate, 

 and the action current of the retina (directed 



from the inner surface toward the rods and cones) undergoes a positive varia- 

 tion. The significance of the first of these changes we shall discuss briefly on 

 page 541 ; of that of the second we know little more than what is contained 

 in the statement that it furnishes us some external indication of an excitation 

 process ; the third goes perhaps a step farther and brings the mode of response 

 of the visual apparatus into line with other forms of protoplasmic activity. 

 But if we wish to know the course of the excitation process we must rely for 

 the most part on our own subjective experiences. 



To how great an extent these subjective phenomena are caused by processes 

 going on in the retina itself, or by changes in the many other parts of the 

 nervous system necessary for the production of a conscious visual sensation, 

 nothing definite can be said (except with regard to certain details). Unless 

 otherwise expressly stated the facts and discussion in what follows relate to 

 the entire nervous mechanism concerned in visual sensation. 



When light falls upon the retina the resulting sensation does not reach its 

 full strength immediately, and similarly vice versa when light is suddenly 



