COLOUR VISION. 171 



The other way in which two samples of the same colour may 

 differ is in the proportion of white light which they contain. A 

 colour which contains no white light at all is, physically speaking, 

 completely SATURATED or full; the more white one mixes with it 

 the less saturated, or the paler, it becomes. 



Experiment 67. Take two sheets of yellow tissue paper and put 

 one over a piece of heavy paper of the same shade and lay 

 the other over a sheet of white paper. They both have the same 

 colour but the first is fuller or more saturated than the second, 

 .paler one. 



The evidence which we have examined so far would lead one 



to think that the sensation of a given colour was entirely dependent 



on the action on the retina of waves of a certain length. It would 



seem that the sensation of yellow, for instance, was simply the 



physiological result of the action of waves from that particular part 



of the spectrum. The student's attention however is directed in 



this regard to the results of the two following experiments. 



Experiment 68. The Effect of Stimulating the Retina with 



Two Different Colours at the Same Time. From the paper 



discs provided choose an orange and a green, colours 



situated immediately on either side of yellow in the spectrum, 



the waves of one being longer and those of the other shorter 



than the yellow waves. Arrange them in the rotator so that 



about half of each shows and spin them rapidly until they seem 



to fuse.* The colour which together they produce is a yellow 



*The ideal way to mix colours on the retina is to cause the rays from those 

 parts of the spectrum which contain the desired colours to converge again, so that 

 they actually fall on the retina at the same time. A simpler means is that adopted 

 here, in stimulating the retina with alternate flashes of the colours following each 

 other in very rapid succession. Advantage is taken of the fact that the reaction 

 of the retina does not cease immediately the stimulus is withdrawn but lasts for a 

 fraction of a second after it. The result cannot be got by mixing together pig- 

 ments of the colours which it is desired to combine. For instance, although blue 

 and yellow light thrown together on the retina give white or grey, blue and yellow 

 pigments mixed give a green colour. The reason for this is that both blue pigment 

 and yellow pigment reflect, along with their particular colours, some green rays. 

 When they are mixed the blue absorbs yellow light which the yellow pigment 

 would have reflected, the yellow pigment does as much for the blue, and the only 

 colour left over is the green, which was completely absorbed by neither. 



