CHAP, in.] SIGHT. 893 



a slight extent the red rays, but reflects the yellow rays and 

 with these many of the green rays ; indigo on the other hand 

 absorbs the red and yellow but reflects the blue and a good 

 deal of the green. Hence when we look at a yellow gamlx^,. 

 patch our retina is excited not by those rays alone which form 

 the yellow of the spectrum, but by many other rays as well ; 

 the colour is not a ' pure ' colour, does not correspond to one 

 of the colours of the spectrum, but is a mixture of more than 

 one. And this is the case with most pigments ; hence when 

 they are employed in experiments on the mixture of sensa- 

 tions, difficulties and even errors arise which are avoided 1>\ 

 the use of the colours of the prism. We may here incidentally 

 remark that mixing the sensations excited by looking at pig- 

 ments gives very different results from mixing the pigments 

 themselves. Thus when gamboge and indigo are mixed the 

 mixture is green because the gamboge absorbs the blue and 

 the indigo absorbs the red and yellow, while both reflect the 

 green. We shall see presently that when the sensation excited 

 by gamboge is mixed with the sensation excited by indigo the 

 result is a sensation not of green but of white ; and we shall 

 see why this is. What we have just said with regard to sur- 

 faces coloured with pigments applies also to glasses stained 

 with pigment, it being understood that the colour of stained 

 glass, seen as a transparent object, corresponds to the rays 

 which it does not absorb. When pure pigments, i.e. pigments 

 corresponding as closely as possible to the prismatic colours, 

 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 tlie 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 hafr 

 passed away. The first result is easily reached by the simple 

 method of placing two pieces of coloured paper a little dis- 

 tance apart on a table, one on each side of a glass plate in- 

 clined 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 dis- 

 tance between the pieces of paper. In the second method, the 

 'colour top' is used ; sectors of the colours to be investigated 

 are placed on a disc made to rotate very rapidly, and the image 

 of one colour is thus brought to bear on the retina so soon 

 after the image of another that the two sensations are fused 

 into one. 



560. When by any of the above methods sensations corre- 

 sponding to the red and yellow of the spectrum are mixed 

 together in certain proportions the result is a sensation of 

 orange, quite indistinguishable from the orange of the spec- 



