246 PRACTICAL PHYSIOLOGY 



sensations to fusion of certain simpler sensations, which are described 

 as primary colour sensations. In the Young-Helm lioltz theory the 

 primary sensa ions are those corresponding to red, green, and blue- 

 violet; in the tiering theory they are grouped in pairs, which are the 

 red and green sensations, the yellow and blue sensations, and the 

 white and black sensations. It is necessary to assume the existence 

 of certain photo-chemical substances in the retina, which can be acted 

 upon by the light of the primary colours. The light at the ends of the 

 spectrum would, in accordance with the Young Helmholtz theory, act 

 upon either the red visual substance or the violet visual substance, in 

 the intermediate part of the spectrum upon all three visual substances 

 to different extents. If all are affected more or less equally, the com- 

 pound sensation of white is produced. 



In the Bering theory there would also be assumed to exist three 

 primary visual substances, but according to the chemical changes in any 

 single substance, whether of the constructive or destructive variety, so 

 a sensation corresponding to one of the complementary colours of the 

 different pairs would be brought about. 



A certain classification of colours is necessary. They may be con- 

 veniently described as varying in hue, tint, or shade. The hue of 

 a colour is its colour tone, corresponding to its wave length. The 

 tint of a colour depends upon its purity, or whether it is admixed with 

 white in other words, depends upon its saturation. The shade of a 

 colour is an expression of its brightness or intensity, or, what comes to 

 much the same thing, the degree to which it is admixed with black. 



1. Colour Tone. In reviewing the changes of hue that are appreciable 

 in examining the spectrum, it is to be noticed that the changes do not 

 occur at any regular intervals corresponding to wave lengths. Changes 

 of colour tone are most easily appreciated in the yellow, green, and blue 

 green. At the red end and violet ends there appears to be little or no 

 change of hue. 



The variations in saturation or tint can be seen by using the red and 

 white discs of a colour mixed in varying proportions and noting the 

 corresponding sensations produced. 



2. Intensity. Variations in intensity cause changes in the quality of 

 colours. At their maximum brightness colours tend to give the sensa- 

 tion of white, though they never completely do this. The yellow will 

 the most easily ; the blue and violet approach close to it. The red is 

 most distant in producing the sensation of white. 



EXPERIMENT I. Take a small square of red paper and a similar 

 piece of blue paper which in a light of moderate brightness appear of 

 approximately equal intensity. Carry these to an almost dark room 



