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 
invariable ? 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 Lee- 
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. series 
