OPTO'METER (from twroaat, to aee, and akrpor, a measure) is an in strument for ascertaining with precision the refractive powers of lenses, and the distances at which minute objects may be distinctly seen. The idea originated with Scheirer ; but such an instrument was con structed by Dr. Porterfield, and was improved by Dr. Thomas Young. It consisted of a slip of ivory, unpolished, or of wood covered with white paper, about eight inches long and half an inch broad, on which, in the direction of its length, was drawn a narrow and well-defined black line. At one extremity of the slip was fixed a plate of ivory, or a piece of card, nearly perpendicular to its length, and this was per forated either with a single aperture or with two apertures at distances from one another varying to of an inch, but not exceeding the diameter of the pupil of the eye. On applying the eye to a single aperture, and looking in the direction of the line, drawn on the instrument, the line appears to have a certain breadth, and to be ill defined, at the nearest extremity : the breadth gradually diminishes at points successively more remote till it becomes a minimum, when the line becomes distinct ; beyond that point the line gradually increases in breadth, becoming again ill defined. On applying the eye to a double aperture, the line appears to be double, the parts seeming to cross one another at a very acute angle, and intersection is at the place where a single line would have had the minimum breadth : beyond this intersection the two lines appear to diverge from one another, and to become indistinct. The point of intersection is that at which • minute object being placed, its image would be distinctly I seen by the eye at the aperture, and its distance from the eye may be immediately found by means of a made of inches on the instrument, a eliding index being, at the time of making the observation, moved to the point of intersection.
This distance affords, evidently, a measure of the refractive power of the observer's eye; since rays of light diverging from a small object at that distance are, by the lenses of the organ, made to converge accurately to a point on the retina, so as to form there • single and well defined image, If the object were either brought nearer to or removed farther from the eye, the rays, after passing through the two apertures, would form two images, both of which would appear to be loss perfectly defined than the single image.
It being assumed that • person whose sight is perfect sees a small object distinctly at the distance of 8 inches from the eye, Dr. Young obtained by computation a aeries of numbers which he placed on the scale of the optometer, at such distances from the plate containing the apertures, that a spectator having • prcallopic eye (or one which, like those of most persons who are advanced in life, has distinct vision of small objects only when they are at greater than 8 inches) on looking through the slits and observing the place where the two images of the line cross each other, might ascertain at once, by the number at the place, the focal length of a convex lens which would enable him to see distinctly at 8 inches. He also computed a series of numbers which he placed on the scale in such situations that a person having a myopic eye, or ono which has distinct vision of objects at less than that distance from it, on observing the place at which the lines appeared to intersect one another, might ascertain the virtual focus of a concave lens which would enable him to see distinctly at 8 inches. The optometer thus became a means of enabling persons to make choice of the proper lenses for spectacles which would correct the imperfections of their natural vision.
But the scale of Dr. Young's optometer being, for convenience, only 8 inches long, and the inferior limit of distinct vision in a presbyopic eye being beyond that distance, Dr. Young applied to his instrument a convex lens of 4 inches focus, by which the point of distinct vision for such an eye was reduced within 8 inches from it : the numbers on his scale were therefore computed for the instrument when furnished with such a lens.
Dr. Young used the optometer for the purpose of determining the refractive powers of the eye, not only in its usual state, but also when, in cases of cataract, the crystalline lens had been displaced or extracted, and when the effect of the cornea had been rendered null by immersing the anterior part of the eye in water, which was contained in a cell of ro a proper form, and having a plane glass in front. By such means be endeavoured to ascertain in what manner the eye accommodates itself to distinct vision for objects at different distances from it ; and his conclusion is that the faculty depends entirely upon changes of figure which take place in the crystalline lens. (Lectures on Natural Philosophy, vol. ii.) .
ORA. (Mosey.]