770 SIGHT. 



of nature result from various admixtures of yellow, red, white, and black ; 

 and a small quantity of white light, scattered over a large area of the retina, 

 i. e., white largely mixed with black, forms a gray. In fact, the qualities of 

 a color depend (1) on the nature of the prismatic color or colors, i.e., on the 

 wave-lengths of the constituent rays, falling on a given area of the retina; 

 (2) on the amount of this colored light which falls on the area of the retina 

 in a given time ; and (3) on the amount of white light falling on the same 

 area at the same time. When rays corresponding to a prismatic color fall 

 upon the retina unaccompanied by any white light, the color is said to be 

 " saturated ;" and a color is spoken of as more or less saturated according 

 as it is mixed with less or more white light. When we are led to describe a 

 color as being of such a tint or hue, we are guided by the first of the above 

 conditions. But we have no common phrases by which we distinguish the 

 second of the above conditions from the third. The word " pale," it is true 

 is most frequently used to express a color very slightly saturated ; but the 

 words " rich " or " deep " are used sometimes as meaning highly saturated, 

 sometimes as meaning simply that a large quantity of light of the particular 

 hue is passing into the eye. So also with the phrase " bright ;" this we 

 often use when a large amount of colored and white light fall at the same 

 time on the same retinal area, but we sometimes also use it to express the 

 mere intensity of the sensation. 



The best method of fusing color sensations is that adopted by Maxwell, of 

 allowing two different parts of the spectrum to fall on the same part of the retina 

 at the same time. The use of the pure prismatic colors eliminates errors which 

 arise when pigments, the colors of which are not pure, but mixed, are employed. 

 And where pigments are used, it is the sensations to which the pigments give rise 

 which must be mixed and not the pigments themselves. Thus while the sensations 

 gained by looking at gamboge yellow and indigo respectively when fused give rise 

 to a sensation of white, gamboge and indigo themselves when mixed appear green. 

 The color of the mixed pigment is due to the fact that the rays which reach the eye 

 from the mixture are those which are least absorbed by the two pigments. The 

 gamboge absorbs the blue rays very largely, but the green to a much less extent ; 

 while the indigo absorbs the red and yellow rays very largely, but also absorbs very 

 little of the green. Hence green is the predominant hue of the mixture. When 

 pure pigments, i. e. , pigments corresponding as closely as possible to the prismatic 

 colors, are used, satisfactory results may be gained, either by using the reflected 

 image of one pigment, and arranging so that it falls on the retina at the same spot 

 as the direct image of the other pigment, or by allowing the image of one pigment 

 to fall on the retina before the sensation produced by the other has passed away. 

 The first result is easily reached by Helmholtz's simple method of placing two 

 pieces of colored paper a little distance apart on a table, one on each side of a glass 

 plate inclined at an angle. By looking with one eye down on the glass plate the 

 reflected image of the one paper may be made to coincide with the direct image of 

 the other, the angle which the glass plate makes with the table being adjusted to 

 the distance between the two pieces of paper. In the second method, the " color 

 top " is used ; sectors of the colors to be investigated are placed on a disc made to 

 rotate very rapidly, and the image of one color is thus brought to bear on the 

 retina so soon after the image of another that the two sensations are fused into 

 one. 



662. When the sensation corresponding to the several prismatic colors 

 are fused together in various combinations, the following remarkable results 

 are brought about : 



1. When red and yellow in certain proportions are mixed together the 

 result is a sensation of orange, quite indistinguishable from the orange of the 

 spectrum itself, Now the latter is produced by rays of certain wave-lengths, 

 whereas the rays of red and of yellow are respectively of quite different 

 wave-lengths. The orange of the spectrum cannot be made up by any mix- 



