306 RECENT PROGRESS OF THE THEORY OF VISION. 



way, whatever be the quality of the impressions. By 

 this means we learn to recognise such changes as be- 

 longing to the special phenomena which we call changes 

 in space. This is enough for the object of Empirical 

 Philosophy, and we need not further enter upon a dis- 

 cussion of the question, how much of universal concep- 

 tions of space is derived a priori, and how much a 

 posteriori ? l 



An objection to the empirical Theory of Vision might 

 be found in the fact that illusions of the senses are 

 possible ; for if we have learnt the meaning of our 

 sensations from experience, they ought always to agree 

 with experience. The explanation of the possibility of 

 illusions lies in the fact that we transfer the notions 

 of external objects, which would be correct under normal 

 conditions, to cases in which unusual circumstances have 

 altered the retinal pictures. What I call ' observation 

 under normal conditions ' implies not only that the rays of 

 light must pass in straight lines from each visible point 

 to the cornea, but also that we must use our eyes in the 

 way they should be used in order to receive the clearest 

 and most easily distinguishable images. This implies 

 that we should successively bring the images of the 

 separate points of the outline of the objects we are 

 looking at upon the centres of both retinae (the yellow 

 spot), and also move the eyes so as to obtain the surest 

 comparison between their various positions. When- 

 ever we deviate from these conditions of normal vision, 

 illusions are the result. Such are the long recognised 

 effects of the refraction or reflection of rays of light 

 before they enter the eye. But there are many other 



1 The question of the origin of our conceptions of space is discussed by 

 Mr. Bain on empirical principles in his treatise on The Senses and the In- 

 tellect, pp. 114-118, 189-194, 245, 363-392, &c. TR. 



