120 THE THEORY OF COLOURS IN RELATION TO COLOUR-BLINDNESS. 



do so by artificial means; and our reasonings on them must be guided by some 

 theory. 



The most general form in which the existing theory can be stated is this, 



There are certain sensations, finite in number, but infinitely variable in 

 degree, which may be excited by the different kinds of light. The compound 

 sensation resulting from all these is the object of consciousness, is a simple act 

 of vision. 



It is easy to see that the number of these sensations corresponds to what 

 may be called in mathematical language the number of independent variables, of 

 which sensible colour is a function. 



This will be readily understood by attending to the following cases : 



1. When objects are illuminated by homogeneous yellow light, the only 

 thing which can be distinguished by the eye is difference of intensity or 

 brightness. 



If we take a horizontal line, and colour it black at one end, with increasing 

 degrees of intensity of yellow light towards the other, then every visible object 

 will have a brightness corresponding to some point in this line. 



In this case there is nothing to prove the existence of more than one 

 sensation in vision. 



In those photographic pictures in which there is only one tint of which 

 the different intensities correspond to the different degrees of illumination of the 

 object, we have another illustration of an optical effect depending on one variable 

 only. 



2. Now, suppose that different kinds of light are emanating from different 

 sources, but that each of these sources gives out perfectly homogeneous light, 

 then there will be two things on which the nature of each ray will depend : 

 (1) its intensity or brightness; (2) its hue, which may be estimated by its 

 position in the spectrum, and measured by its wave length. 



If we take a rectangular plane, and illuminate it with the different kinds 

 of homogeneous light, the intensity at any point being proportional to its hori- 

 zontal distance along the plane, arid its wave length being proportional to its 

 height above the foot of the plane, then the plane will display every possible 

 variety of homogeneous light, and will furnish an instance of an optical effect 

 depending on two variables. 



