.7''. COLOUR VISION. 



falls, just as the pattern on the web formed by a Jacquard loom depends on the 

 mode in which the perforated cards act on the system of moveable rods in that 

 machine. In the eye we have on the one hand light falling on this wonderful 

 structure, and on the other hand we have the sensation of sight. We cannot 

 compare these two things ; they belong to opposite categories. The whole of 

 Metaphysics lies like a great gulf between them. It is possible that discoveries 

 in physiology may be made by tracing the course of the nervous disturbance 



"Up the fine fibres to the sentient brain;" 



but this would make us no wiser than we are about those colour-sensations 

 which we can only know by feeling them ourselves. Still, though it is impossible 

 to become acquainted with a sensation by the anatomical study of the organ 

 with which it is connected, we may make use of the sensation as a means of 

 investigating the anatomical structure. 



A remarkable instance of this is the deduction of Helmholtz's theory of 

 the structure of the retina from that of Young with respect to the sensation 

 of colour. Young asserts that there are three elementary sensations of colour; 

 Helmholtz asserts that there are three systems of nerves in the retina, each 

 of which has for its function, when acted on by light or any other disturbing 

 agent, to excite in us one of these three sensations. 



No anatomist has hitherto been able to distinguish these three systems of 

 nerves by microscopic observation. But it is admitted in physiology that the 

 only way in which the sensation excited by a particular nerve can vary is by 

 degrees. of intensity. The intensity of the sensation may vary from the faintest 

 impression up to an insupportable pain ; but whatever be the exciting cause, 

 the sensation will be the same when it reaches the same intensity. If this 

 doctrine of the function of a nerve be admitted, it is legitimate to reason from 

 the fact that colour may vary in three different ways, to the inference that 

 these three modes of variation arise from the independent action of three differ- 

 ent nerves or sets of nerves. 



Some very remarkable observations on the sensation of colour have been 

 made by M. Sigmund Exner in Professor Helmholtz's physiological laboratory at 

 Heidelberg. While looking at an intense light of a brilliant colour, he exposed 

 his eye to rapid alternations of light and darkness by waving his fingers before 

 his eyes. Under these circumstances a peculiar minute structure made its 

 appearance in the field of view, which many of us may have casually observed. 



