296 THOMAS YOUNG. 



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

 that there was still the same faculty of seeing at different 

 distances perfectly preserved. The second of three pos- 

 sible suppositions, that of an alteration in the dimensions 

 of the whole organ, was 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 whole eye are inva- 

 riable, it must follow that the form of the crystalline is 

 ^variable ? Young, however, did not stop there ; he 

 proved directly, by the minute phenomena of the changes 

 in the images, that the crystalline really changes its cur- 

 vature ; he invented, or at least, gave perfection to, an 

 instrument susceptible of being employed even by the 

 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 distances.* 



* This instrument, called an " Optometer," was originally proposed 

 by Dr. Porterfield, and consists of a simple and ingenious contrivance 

 for ascertaining the focal length of the eye, which varies so greatly in 

 different individuals, and often in two eyes of the same person, and in 

 the same eye under different conditions. Dr. Young greatly improved 

 upon the original construction. It will be found described in the Lec- 

 tures 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 apertures close to the eye, 

 presents only a single image, while in other positions it shows two 

 images. Translator. 



