228 RECENT PROGRESS OF THE THEORY OF VISION. 



imperfect transparency of the media, and all the other 

 defects of which I have spoken. 



The adaptation of the eye to its function is, therefore, 

 most complete, and is seen in the very limits which 

 are set to its defects. Here the result which may be 

 reached by innumerable generations working under the 

 Darwinian law of inheritance, coincides with what the 

 wisest Wisdom may have devised beforehand. A sensible 

 man will not cut firewood with a razor, and so w^e may 

 assume that each step in the elaboration of the eye must 

 have made the organ more vulnerable and more slow in 

 its development. We must also bear in mind that soft, 

 watery animal textures must always be unfavourable and 

 difficult material for an instrument of the mind. 



One result of this mode of construction of the eye, of 

 which we shall see the importance bye and bye, is tliat 

 clear and complete 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 limitation, will after- 

 wards come under our notice. 



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

 derstanding 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 sensa- 

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

 solution of the problem. 



