COLOUR VISION. 



tBMPOfr It differ* from the greater part of what is called mental science in 

 the Urge uae which it makes of the physical sciences, and in particular of 

 optics and anatomy. But it gives evidence that it is a mental science by the 

 numerous illustrations which it furnishes of various operations of the mind. 



In this place we always feel on firmer ground when we are dealing with 

 pin-weal science. I shall therefore begin by shewing how we apply the dis- 

 coveries of Newton to the manipulation of light, so as to give you an oppor- 

 tunity of feeling for yourselves the different sensations of colour. 



Before the time of Newton, white light was supposed to be of all ki 

 things the purest. When light appears coloured, it was supposed to have 

 baoome contaminated by coming into contact with gross bodies. We may still 

 think white light the emblem of purity, though Newton has taught us that 

 its purity does not consist in simplicity. 



We now form the prismatic spectrum on the screen [exhibited]. These are 

 the simple colours of which white light is always made up. We can distin. 

 a great many hues in passing from the one end to the other; but it is when 

 we employ powerful spectroscopes, or avail ourselves of the labours of those 

 who have mapped out the spectrum, that we become aware of the immense 

 multitude of different kinds of light, every one of which has been the object 

 of special study. Every increase of the power of our instruments increas- 

 the same proportion the number of lines visible in the spectrum. 



All light, as Newton proved, is composed of these rays taken in difl'i 

 proportions. Objects which we call coloured when illuminated by white light, 

 make a selection of these rays, and our eyes receive from them only a 

 I' the light which falls on them. But if they receive only the pure rays of 

 u single colour of the spectrum they can appear only of that colour. It I 

 place this disk, containing alternate quadrants of red and green paper, in the 

 red rays, it appears all red, but the red quadrants brightest. If I place it in 

 the green rays both papers appear green, but the red paper is now the dan 

 This, then, is the optical explanation of the colours of bodies when illuminated 

 with white light. They separate the white light into its component parts, 

 absorbing some and scattering others. 



Here are two transparent solutions [exhibited]. One appears yellow, it 

 contains bichromate of potash; the other appears blue, it contains sulphate of 

 copper. If I transmit the light of the electric lamp through the two solutions 

 at once, the spot on the screen appears green. By means of the spectrum we 



