strength of light which proceeds from, or is reflected by the object itself, that we can conclude upon the distance at which any particular body will be visible. The power of seeing objects at a distance is very rarely equal in both eyes. When this inequality is great, the person so circumstanced generally shuts that eye with which he sees the least, and employs the other with all its power, and which is one cause of squinting. The object does not appear doubly distinct, by both eyes being placed upon it although they are equally strong, but has frequently been proved not to exceed a 13th part more than if beheld with one; and this is supposed to arise from the two optic nerves uniting near the place they came out of the skull and then separating by an obtuse angle before they enter the eyes; but as the motion made by the impression of objects cannot pass to the brain without passing this united part, the two motions must therefore be combined, and, consequently, cannot act with that force as though they were distinct; but from repeated experiments seem to bear the proportion above stated. There are many reasons to suppose that short-sighted persons see objects larger than others; and it is a certain truth that they see them less. I am myself short-sighted, and my left-eye is stronger than my right. A thousand times have I experienced, upon looking at any object, as the letters of a book, that they appear least to the weakest eye; and that, when I place the book so that the letters appear double, the images of the left-eye, are greater than those of the right. Several others, I have examined, who were in similar circumstances, and I have always found, that the eye which saw every object best, saw it also largest. This may be ascribed to particular habits; for near-sighted people being accustomed to approach close to the object, and to view but a small part of it at a time, they acquire a small standard for magnitude, and when the whole of the object is seen, it necessarily appears smaller to them than to others, whose vision is more enlarged.