There have been many instances of persons becoming short-sighted on a sudden, therefore attributing it to the roundness or prominence of the eye is by no means certain. Mr. Smith, in his Optics, speaks of a young man that became short-sighted as he quitted a cold bath, and who was under the necessity of using a concave glass all his life after; and it cannot be supposed that the vitreous humours were instantly inflated so as to cause this difference in vision. Short-sightedness may arise from the position of the various parts of the eye, especially the retina, from a less degree of sensibility in the retina, or the smallness of the pupil. In the two first cases a concave glass may be used to advantage, but yet objects will not be seen so far, or so distinct, through these glasses as others will perceive with the eye alone, for as short-sighted persons see objects in a diminished form, the concave glass diminishes them still farther. Infants having their eyes smaller than those of adults, must of consequence, see objects smaller also. For as the image formed on the back of the eye must be large, as the eye is capacious, so infants, having it not so great, cannot have so large a picture of the object. This may likewise be a reason, why they are unable to see so distinctly, or at such distances, as persons who have attained the years of maturity, for as objects appear less they must sooner become invisible. Old people see bodies close to them very indistinctly, but bodies at a great distance from them with more precision, than young ones. This may happen from an alteration in the coats, or perhaps the humours of the eye; and not, as is supposed, entirely from their diminution. The cornea, for instance, may become too rigid to adapt itself, and take a proper convexity for seeing near objects, as a flatness must be occasioned by drying that will be sufficient of itself to render their eyes more calculated for distant vision. Although clear and distinct are frequently confounded by writers on optics, yet they are very different; for we may be said, for instance, to clearly see a tower, as soon as we get a view of it, but we must approach near enough to distinguish its component parts before we see it distinctly. Men in years see clearly, but not distinctly; they can discern large bodies at a distance, but cannot distinguish small objects, as the characters in a book, without the help of magnifying glasses. On the contrary, short-sighted people see small objects distinctly, but need the aid of concave glasses to reduce large ones. Much light is also necessary for clear sight, while a small quantity is sufficient for distinct vision.