202 RECENT PROGRESS OF THE THEORY OF VISION. 



we shall see the importance bye and bye, is that clear and com- 

 plete apprehension of external objects by the sense of sight is 

 only possible when we direct our attention to one part after 

 another of the field of vision in the manner partly described 

 above. Other conditions, which tend to produce the same limit- 

 ation, will afterwards come under our notice. 



But, apparently, we are not yet come much nearer to under- 

 standing sight. We have only made one step : we have learnt 

 how the optical arrangement of the eye renders it possible to 

 separate the rays of light which come in from all parts of the 

 field of vision, and to bring together again all those that have 

 proceeded from a single point, so that they may produce their 

 effect upon a single fibre of the optic nerve. 



Let us see, therefore, how much we know of the sensations 

 of the eye, and how far this will bring us towards the solution 

 of the problem. 



II. THE SEXSATION OF SIGHT. 



IN the first section of our subject we have followed the course 

 of the rays of light as far as the retina, and seen what is the 

 result produced by the peculiar arrangement of the optical 

 apparatus. The light which is reflected from the separate 

 illuminated points of external objects is again united in the 

 sensitive terminal structures of separate nerve fibres, and thus 

 throws them into action without affecting their neighbours. 

 At this point the older physiologists thought they had solved 

 the problem , so far as it appeared to them to be capable of solu- 

 tion. External light fell directly upon a sensitive nervous 

 structure in the retina, and was, as it seemed, directly felt there. 

 But during the last century, and still more during the 6rst 

 quarter of this, our knowledge of the processes which take 

 place in the nervous system was so far developed, that Johannes 

 Miiller, as early as the year 1826, 1 when writing that great 

 work on the ' Comparative Physiology of Vision,' which marks 



1 The year in which he was appointed Extraordinary Professor of Phy- 

 siology in the University of Bonn. 



