VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 405 



Most writers on this subject refer this power of the eye to the contraction and 

 dilatation of the iris. Within certain limits this would, on first examination, as 

 already observed, appear to be the case, since the pupil enlarges as the object is 

 farther removed from the eye, and again contracts as it is brought near. The 

 extent of this principle I have already pointed out; but I suspect we also err in 

 attributing to the difference of distance what are only effects of different quan- 

 tities of light, a circumstance in which it is the more easy to commit error, as 

 they are generally proportionate one to the other; i. e. as the object is near we 

 require a less degree of light, and to exclude what is superfluous the iris con- 

 tracts; but as it is more distant, a greater quantity of light becomes necessary, 

 and the iris dilates: thus far we see the use of the enlargement or diminution of 

 the pupil, as the object is more or less distant. But distinct vision does not 

 consist in the quantity of light alone, though too much or too little would ob- 

 scure the image. It is also necessary that the rays which flow from the object 

 should fall on the retina in a certain direction, to form a distinct picture; but 

 surely the greater or less quantity of light, the greater or less number of rays, 

 which it is only the property of the iris to diminish or increase, cannot alter the 

 direction. 



But there is still another argument, to prove that the contraction or enlarge- 

 ment of the pupil is not of itself sufficient to produce distinct vision at different 

 distances, viz. that the myopes, whose pupil contracts and dilates as in other 

 eyes, are still unable to adapt the eye to different distances; and the means by 

 which this is remedied certainly does not consist in a larger or smaller aperture 

 for the rays to pass through, but a power of altering their direction, which the 

 change in the shape of the eye had rendered too convergent. The same fact is 

 also observable in those who squint; the pupil in both eyes equally contracts 

 and dilates, but still the vision of one eye is less perfect than the other. Ano- 

 ther principle on which it has been attempted to explain this power of the eye, 

 is a supposed change in the convexity of the crystalline lens; the ancients had 

 some obscure notions of it, but it has been lately pursued by Mr. Thomas 

 Young, in a paper published in the Philos. Trans, for 1793, (p. 318, of this vol.) 

 He has endeavoured to demonstrate the existence of muscles in the crystalline 

 lens, and by their action to account for distinct vision at different distances. 

 This opinion deserves here the more particular examination, having met the at- 

 tention of the K. s.j and so may be likely to influence the general opinion on 

 this subject. 



That we may not mistake the meaning of the author, I beg leave to premise 

 his description of the structure of the lens. " The crystalline lens of the ox," 

 &c. p. 320, of this vol. In the first place, to say nothing of the transparency 

 of muscles, as an argument against the existence, we must unavoidably suppose. 



