Vision, tracitig them to Ftinctional Actions of the Brain. 347 



image of equal or superior brilliancy produces no such effect: 

 besides, it has been shown above, that to ascribe the state of 

 the sensibility in and around the part of the retina where the 

 bright light falls, to the physical impulse of the difference be- 

 tween a distinct and an indistinct image, would be to assert a 

 physical absurdity ; it follows unavoidably, therefore, that the 

 changes in the sensibility in the exposed eye arise from actions 

 ab interno. With regard to the state of the sensibility in the un-' 

 exposed eye in one of these affections, there cannot be two opi- 

 nions : it must be produced by an action from within ; and 

 though we are not yet prepared to say in what manner, or by 

 what medium, an action of the brain can affect the sensibility 

 to light, yet the fact is a most important one; and the perfectly 

 correct manner in which the defect of sensibility in the exposed 

 eye is balanced by an excess of sensibility in the unexposed 

 eye, not only affords an additional argument for their common 

 origin, but seems to open a path, which, if duly followed, may 

 lead to interesting discoveries in this obscure department of 

 physiology. In the other affection of sight, though the changes 

 in the sensibility are confined to the exposed eye only, yet the 

 same correct balance of excess and defect is found to exist, — a 

 circumstance that strongly corroborates the conclusion, that 

 both of these remarkable affections of vision are produced by 

 one common principle, and arise from one common seat. But, 

 in the second place, the exciting causes of these affections are 

 of such a nature, I should humbly submit, as to render them 

 totally incapable of producing any hut functional actions pre- 

 organized for the occasion. Indistinctness o/'mcge (disregard- 

 ing its physical causes, which have been shown to be inade- 

 quate to the effects,) is a purely negative quality, and can have 

 no effects beyond the simple perception of it, except in so far 

 as distinct vision is the end to which the mechanism of its 

 several organs, viz. the eye, optic nerve, and brain, has been 

 adapted : that of these three the brain is the directing organ 

 in another highly interesting function of vision, which is also 

 excited into action by indistinctness of image, admits of another 

 kind of proof: the function I mean is that by which the eyes are 

 accommodated to the different distances of objects. Suppose, 

 after looking at a distant object, that we wish to view one near 

 at hand, what we do in this case is to direct the two eyes so as 

 to make their axes meet in the object to be viewed. If the 

 former accommodation of the eyes continued, the object now 

 viewed would be indistinct ,- but to prevent this, an action of 

 the brain produces a change in the eye not yet sufficiently un- 

 derstood, by which a correct image of the object is formed on 

 the retina. That the unknown change here mentioned is pro- 

 2 Y 2 



