226 RECENT PROGRESS OF THE THEORY OF VISION. 



ther details, to understand which it would be necessary to enter 

 upon lengthy descriptions of many separate experiments. 



"We have already seen enough to answer the question 

 whether it is possible to maintain the natural and innate convic- 

 tion that the quality of our sensations, and especially our sensa- 

 tions of sight, give us a true impression of corresponding 

 qualities in the outer world. It is clear that they do not. 

 The question was really decided by Johannes Miiller's deduction 

 from well-ascertained facts of the law cf specific nervous energy. 

 Whether the rays of the sun appear to us as colour, or as 

 warmth, does not at all depend upon their own properties, but 

 simply upon whether they excite the fibres of the optic nerve, 

 or those of the skin. Pressure upon the eyeball, a feeble current 

 of electricity passed through it, a narcotic drug carried to the 

 retina by the blood, are capable of exciting the sensation of 

 light just as well as the sunbeams. The most complete differ- 

 ence offered by our several sensations, that namely between those 

 of sight, of hearing, of taste, of smell, and of touch this 

 deepest of all distinctions, so deep that it is impossible to draw 

 any comparison of likeness, or unlikeness, between the sensations 

 of colour and of musical tones does not, as we now see, at all 

 depend upon the nature of the external object, but solely upon 

 the central connections of the nerves which are affected. 



We now see that the question whether within the special 

 range of each particular sense it is possible to discover a coin- 

 cidence between its objects and the sensations they produce, is 

 of only subordinate interest. What colour the waves of ether 

 shall appear to us when they are perceived by the optic nerve 

 depends upon their length. The system of naturally visible 

 colours offers us a series of varieties in the composition of light, 

 but the number of those varieties is wonderfully reduced from 

 an unlimited number to only three. Inasmuch as the most im- 

 portant property of the eye is its minute appreciation of locality, 

 and as it is so much more perfectly organised for this purpose 

 than the ear, we may be well content that it is capable of re- 

 cognising comparatively few differences in quality of light; the 

 ear, which in the latter respect is so enormously better provided, 



