COLOUR VISION. 



in 



hi* Lectume* Optica, where he shews how to take a beam of light, to 

 separate it into its components, to deal with these components as we please by 

 means of slita, and afterwards to unite them into a beam again. The observer 

 looks into the box through a small slit. He sees a round field of light con- 

 sisting of two semicircles divided by a vertical diameter. The semicircle on the 

 left consists of light which has been enfeebled by two reflexions at the surface 

 of glass. That on the right is a mixture of colours of the spectrum, the positions 

 and intensities of which are regulated by a system of slits. 



The observer forms a judgment respecting the colours of the two semicircles. 

 Suppose he finds the one on the right hand redder than the other, he says so, 

 and the operator, by means of screws outside the box, alters the breadth of 

 one of the slits, so as to make the mixture less red; and so on, till the right 

 semicircle is made exactly of the same appearance as the left, and the line of 

 separation becomes almost invisible. 



When the operator and the observer have worked together for some time, 

 they get to understand each other, and the colours are adjusted much more 

 rapidly than at first. 



When the match is pronounced perfect, the positions of the slits, as indicated 

 by a scale, are registered, and the breadth of each slit is carefully measured by 

 means of a gauge. The registered result of an observation is called a " colour 

 equation." It asserts that a mixture of three colours is, in the opinion of the 

 observer (whose name is given), identical with a neutral tint, which we shall call 

 Standard White. Each colour is specified by the position of the slit on the 

 scale, which indicates its position in the spectrum, and by the breadth of the 

 slit, which is a measure of its intensity. 



In order to make a survey of the spectrum we select three points for 

 purposes of comparison, and we call these the three Standard Colours. The 

 standard colours are selected on the same principles as those which guide the 

 engineer in selecting stations for a survey. They must be conspicuous and 

 invariable and not in the same straight line. 



In the chart of the spectrum you may see the relations of the various 

 colours of the spectrum to the three standard colours, and to each other. It 

 is manifest that the standard green which I have chosen cannot be one of the 

 true primary colours, for the other colours do not all lie within the triangle 

 formed by joining them. But the chart of the spectrum may be described as con- 

 sisting of two straight lines meeting in a point. This point corresponds to a green 



