VISUAL SENSATIONS. 769 



and yet only one sensation would be produced. So also in the less sensitive 

 peripheral parts of the retina two points of the retinal image might stimu- 

 late two cones a considerable distance apart, and yet give rie to one sensa- 

 tion only. 



In the case where the two points lie entirely within the area of a single 

 cone, it is exceedingly probable that, even if the adjacent cones or cone- 

 fibres in the retina are not at the same time stimulated, impulses radiate 

 from the cerebral ending of the excited cone into the neighboring cerebral 

 endings of the neighboring cones ; in other words, the sensation-area in the 

 brain does not exactly correspond to and is not sharply defined like the 

 retinal area, but gradually fades away into neighboring sensation-areas. 

 We may imagine two points of the retinal image so far apart that even the 

 extreme margins of their respective cerebral sensation-areas do not touch 

 each other in the least ; in such a case there can be no doubt about the two 

 points giving rise to two sensations. We might, however, imagine a second 

 case where two points were just so far apart that their respective sensation- 

 areas should coalesce at their margins, and yet that, in passing from the 

 centre of one sensation -area to the centre of the other, we should find on 

 examination a considerable fall of sensation at the junction of the two 

 areas ; and in a third case we might imagine the two centres to be so close 

 to each other that in passing from one to the other no appreciable diminu- 

 tion of sensation could be discovered. In the last case there would be but 

 one sensation, in the second there might still be two sensations if the mar- 

 ginal fall were great enough, even though the areas partially coalesced. 

 Thus, though the mosaic of rods and cones is the basis of distinct vision, the 

 distinction or fusion of two vision impulses is ultimately determined by the 

 disposition and condition of the cerebral centres. Hence the possibility of 

 increasing by exercise the faculty of distinguishing two sensations, since by 

 use the cerebral sensation-areas become more and more differentiated. This, 

 however, is even more strikingly shown in touch than in sight. 



Color Sensations. 



661. When we allow sunlight reflected from a cloud or sheet of paper 

 to fall into the eye, we have a sensation which we call a sensation of white 

 light. When we look at the same light through a prism, and allow differ- 

 ent parts of the spectrum to fall in succession into the eye, we have sensa- 

 tions which we call respectively sensations of red, orange, yellow, green, blue, 

 violet, etc., light. In other words, rays of light falling on the retina give 

 rise to different sensations, according to the wave-lengths of the rays. 

 Though we speak of the spectrum as consisting of a few colors, such as red, 

 orange, etc., there are an almost infinite number of intermediate tints in the 

 spectrum itself; and we perceive in external nature a large number of colors, 

 such as purple, brown, gray, etc., which do not correspond to any of the 

 color sensations gained by regarding the successive parts of the spectrum. 

 We find, however, on examination, that certain distinct color sensations, not 

 corresponding to any of the colors of the spectrum may be obtained by the 

 fusion of the sensations caused by two or more of the prismatic colors. 

 Thus purple, which is not present in the spectrum, may be at once produced 

 by fusing the sensations of blue and red in proper proportions. Moreover, 

 many of the various tints and shades of nature may be imitated by fusing a 

 particular color sensation with the sensation of white, or by allowing a cer- 

 tain quantity of light of a particular color to fall sparsely over the area of 

 the retina, which is at the same time protected from the access of any other 

 light, i. e., as we say, by mixing the color with black. Thus the browns 



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