CHAP, ii.] SIGHT. 525 



falling on the same area at the same time. When rays corre- 

 sponding to a prismatic colour fall upon the retina unaccompanied 

 by any white light, the colour is said to be 'saturated'; and 

 a colour 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 colour 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 colour 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 coloured 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 colour 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 

 colours eliminates errors which arise when pigments, the colours 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 bv 

 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 colour 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 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 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 

 coloured paper a little distance apart on a table, one on each side of a 

 glass plate inclined at an angle. By locking 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 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. 



When the sensations corresponding to the several prismatic 



