298 THOMAS YOUNG. 



INTERFERENCES. 



The most beautiful discovery of Young, that which will 

 render his name imperishable, was suggested to him by 

 an object in appearance very trivial ; by those soap bub- 



the public are superficial, &c. Young may have been, in most of his 

 speculations, too profound for the many ; but this particular instance 

 of the structure of the eye and theory of vision is, perhaps, of all his 

 researches, that which can be the least open to this charge. The sub- 

 ject is not itself abstruse : it is one easily understood by every edu- 

 cated person, without mathematical attainments; and the point at 

 issue was a simple question of fact requiring no profound physiological 

 knowledge to appreciate, whether the crystalline has or has not a mus- 

 cular structure capable of changing its convexity. The real state of 

 the case seems to be very satisfactorily explained by Dean Peacock 

 (p. 36, et sej.), from whose account, as well as from what has been since 

 written, it appears, after all that has been done both by Dr. Young and 

 others, that there is even at the present day considerable difference of 

 opinion on the subject. 



Perhaps the most comprehensive survey of the whole subject which 

 recent investigation has produced will be found in the paper of Pro- 

 fessor J. D. Forbes in the Edin. Transactions, vol. xvi. part I. 1845. 

 After giving a summary view of preceding researches, and adverting 

 to the prevalent opinion among men of science, that the true explana- 

 tion yet remains to be discovered (most anatomists denying as a fact 

 the existence of the muscular structure which Young conceived he had 

 proved), Professor Forbes proposes, as his own view of the cause, the 

 consideration of the remarkable variation in density of the crystalline 

 towards its central part; coats of different density being disposed in 

 different layers, may be acted on by the pressure of the humours of 

 the eye when the external action of the muscles compresses them, and 

 thus increase the curvature of the lens, when the eye is directed to a 

 near object, the whole consistence especially in the outer parts being 

 of a gelatinous or compressible nature, and the central part more solid 

 and more convex. Thus uniform pressure on the outer parts would 

 tend to make the outer parts conform more nearly to the more convex 

 interior nucleus. 



It may be added that many physiologists are of opinion that, after all, 

 there does not exist a sufficient compressive action on the ball of the 

 eye to produce the effect supposed. Translator. 



