DR KNOX on the Comparative Anatomy of the Eye. 45 



suited to him *. I have repeatedly seen this bird deceived, and 

 have ascertained on many occasions, that though birds possess 

 the power of very distant vision, such vision is not distinct. By 

 the point of distinct vision, I understand the distance at which 

 a moderately small object may be clearly and distinctly made 

 out, independently of all experience. It must be obvious to 

 every one, that this distance is very limited, and depends, to a 

 certain extent, on the size of the object. Instead, therefore, of 

 talking of the limits of distinct vision, we shall consider the fa- 

 culty which the eye possesses, of adapting itself to the general 

 perception of objects placed at a variety of distances. We shah 1 

 return to this subject, when describing the means employed in 

 effecting the changes supposed to take place within the eye- 

 ball. 



There is still one other faculty possessed in various degrees 

 of excellence by different individuals ; I allude to rapidity of vi- 

 sion, or the power of instantaneously changing the focus of the 

 eye, and adapting it to objects placed at various distances. This, 

 no doubt, depends on the irritability of the organ, or, perhaps, 

 of the nervous system ; it is connected, as shah 1 be shewn, with 

 the distribution of the ciliary nerves, and is of the same nature 

 with muscular motion in other parts of the body ; it is not pecu- 

 liar to any complexion, and hence will be found to be as com- 

 monly possessed by the blue as by the dark eye. The exercise 

 of this faculty to its greatest extent, is what constitutes the 

 sportsman ; but most men possess it to a considerable extent 

 when roused by danger, or excited by the unexpected appear- 

 ance of an extraordinary object. I have observed remarkable 

 differences amongst individuals in this respect, and shall cite an 



* I have demonstrated, in a brief notice, which, I believe, was inserted in some 

 of the French journals, that it is by sight only that the vulture is led to discover his 

 prey, and not by the sense of smelling. 



