If objects-are seen through a perfectly flat glass, the rays of light pass through it from them to thp eye, in a straight direc tion, and parallel to each other, and con sequently the objects appear very little either diminished or enlarged, or nearer, or further off, than to the naked eye ; but if the glass they are seen through have any degree of convexity, the rays of light are directed from the circumference to wards the centre, in an angle proportion al to the convexity of the glass, and meet in a point, at a greater or lesser distance from the glass, as it is more or less con vex. This point, where the rays meet, is called the focus, and this focus is nearer or further off, according to the convexity of the glass ; for as a little convexity throws it to a considerable distance, so, when the convexity is much, the focus j,s very near. Its magnifying power is also in the *ame'proportion to the convexity ; for as a flat glass scarcely magnifies at all, the less a glass departs from flatness, the less of course it magnifies ; and the more it approaches towards the globular figure, the nearer its focus is, and the more its magnifying power. People's different length of sight depends on the same principle, and arises from more or less convexity of the cornea and crystalline humour of the eye ; the rounder these are, the nearer will the focus or point of meeting rays be, and an ob ject must be brought to see it well. The case of short-sighted people is only an 4:A1'r-roundness of the eye, which makes a very near focus ; and that of old people is a sinking or flattening of the eye, whereby the focus is thrown to a great distance; so that the former may properly be called eyes of too short, and the latter eyes of too long a Rims. Hence, too, the remedy for the last is a convex glass, to supply the want of convexity in the eye itself, and brings the rays, to A shorter fbetea lvhereas a concave glass is need ful for the first to scatter the rays, and prevent their coming to a point too soon. The nearer any object can be brought to the eye, the larger will be the angle tin der which it appears, and the more it will be magnified. Now, that distance
from the naked eye, where the gene rality of peoplb are supposed to see small objects best, is about six inches; consequently, when such objects are brought nearer than six inches, they will become less distinct ; and if to four or three, they will scarcely be seen at all. But by the help of convex glasses we are enabled to view things clearly at much shorter distances than these ; for the nature of a convex lens is to render an object distinctly visible to the eye at the distance of its focus ; wherefore the smaller a lens is, and the more its con vexity, the nearer is its focus, and the more its magnifying power. Now, it is evident from the figure, that if either the cornea, or crystalline humour, or both of them, be too flat, their focus will not be On the retina, where it ought to be, in order to render vision distinct, but be yond the eye. Consequently those rays which flow from the object, and pass through the humours of the eye, are not sufficiently c'onverged to unite, and there fore the observer can have but a very in distinct view of the object. This is re medied by placing a convex glass, of a proper &ens, before the eye, which makes the rays converge sooner, and imprints the image duly on the retina. If either the cornea or crystalline humour, or both of them, be too convex, the rays that enter in from the object will be con verged to a focus in the vitreous humour, and by diverging from thence to the re tina, will form a very confused image thereon; and so, of course, the observer will have as confused a view of the object, as if his eye had been too flat. This in Convenience is remedied by placing a concave glass before the eye, which grass, by causing the rays to diverge between it and the eye, lengthens the focal dis tance, so that if the glass be properly chosen, the rays will unite at the retina, and fbrm a distinct picture of the object upon it.