118 EULOGY ON THOMAS YOUNG. 



while his labors were as yet unpublished, and not even communicated 

 to any one. However, this point of the discussion soon lost its impor- 

 tance. The learned Leuweuhoek, armed with his powerful microscopes, 

 traced out and gave iigiires of the muscular libres in all their ramilica- 

 tioiis in the ca\\'stalline of a fish. To awaken the attention of the 

 scientific world, tired with these long debates, nothing less was neces- 

 sary than the liigli renown of the two new members of the Eoyal 

 Society who entered the lists — one a celebrated anatomist 5 the other 

 the most eminent instrument-maker of whom England could boast. 

 These jointly presented to the Eoyal Society a memoir, the fruit of their 

 combined labors, intended to establish the complete unalterability of 

 the form of the crystalline. The scientific world was not prepared to 

 admit that Sir Everard Home and liamsden together could possibly 

 make inaccurate experiments, or be deceived in micrometrical measure- 

 ments. Young himself could not believe it, and in consequence he did 

 not hesitate publicly to renounce his theor3\ This readiness to own 

 himself vanquished, so rare in a young man of twenty-five, and 

 especially on the occasion of a first publication, was, in this instance, 

 an act of modesty without example. Young, however, had really 

 notliiug to retract. In 1800, after having withdrawn his former dis- 

 avowal, our colleague developed anew the theory of the change of form 

 of the crystalline in a memoir against which, from that time, no serious 

 objection has been brought. 



isTothing could be more simple than his line of argument ; nothing 

 more ingenious than his experiments. Young, in the first instance, got 

 rid of the hypothesis of a change of curvature in the cornea by the aid 

 of microscopic observations, which were of a kind to render the most 

 minute variations appreciable. We can say more : he placed the eye in 

 special conditions where changes of curvature in the cornea would have 

 been without effect ; he plunged the eye in water, and proved that there 

 was still the same faculty of seeing at difierent distances preserved. 

 The second of three possible suppositions, that of an alteration in the 

 dimensions of tlie whole organ, vras again overthrown by a multitude 

 of objections and of experiments which it was difficult to resist. 



The problem thus seemed finally settled. Who does not see, in fact, 

 that if, of three only possible solutions, two are put out of the question 

 the third is necessarily established ; that if the radius of curvature of 

 the cornea and the longitudinal diameter of the tchole eye are invariable, 

 it must follow that the form of the crystalline is invariable? Young, 

 however, did not stop there; he proved directlj^, by the minute phe- 

 nomena of the (changes in the images, that the crystalline really changes 

 its curvature; he invented, or at least gave perfection to, an instrument 

 susceptible of being employed even by tlie least intelligent persons, and 

 those least accustomed to delicate experiments, and armed with this 

 new means of investigation, he assured himself that those individuals 

 in whose eyes the crystalline has been removed. in the operation for 

 cataract did not enjoy the faculty of seeing equally distinctly at all dis- 

 tances.* 



* This instruuieut, called an " optometer," -^'as originally proposed by Dr. Porterficld, , 

 and consists of a simple and iugenions contrivance for ascertaining the focal length of 

 the eye, which varies so greatly in different individnals, and often in two eyes of the 

 same'person, and in the same eye iinder different conditions. Dr. Yonug greatly im- 

 ])roved npon the original eonstrnction. It will be founu described in the Lectures on 

 Natural Philosophy, vol. ii, p. 576. The principle of it consists in measuring accurately 

 the distance of an object from the eye at which perfectly distinct vision is obtained, 

 and which is determined when the object seen through two small apertiires close to 

 the eye presents only a single image, while in other i)ositious it shows two images. — 



TUANSLATOR. 



