We will suppose that it is the human eye which is to be examined. The room is to be made dark; the person to be seated; a light--the white flame of an oil-lamp or an Argand gas-burner--to be placed near his head, on the side, and at the level of the eye to be observed. The observer takes then the concave mirror in the hand of the side toward the lamp, and placing it against the front of his eye, so that the upper edge rests against his eyebrow, brings his head to the level of that of the person seated, looks through the central perforation at the eye to be observed, and by a little careful change in the direction of the mirror casts, by its aid, upon the eye examined the light of the lamp. He will now perceive that the pupillary aperture is illuminated, and, no longer black, shines with a silvery or reddened light. He takes now the bi-convex lens of short focus in the hand hitherto free, and places it in front of the examined eye, and at such a distance as to make the focus of the lens coincide with the pupil of that eye --distance varying from two to three inches. He himself will usually need to be at a distance from twelve to eighteen inches. This is for normal eyes. The slight movements backward and forward necessary to adjust these distances correctly, are effected very easily and precisely after practice; but at first it is a little difficult to avoid changing the direction of the mirror while thus slightly advancing or retiring the head; and this is a point on which it is well to give a warning, for it is a frequent source of discouragement to beginners, who find that at every movement they interfere with the illumination of the eye, and so suffer from a series of little failures at the outset. The first thing, in fact, that every one sees amounts to a little more than a red, luminous disc; those who begin by seeing nothing more, therefore, need not to be discouraged; a little patience and time will enable them to see what more practised persons describe. The eye to be examined may be more fully observed by dilating the pupil {123} with atropine--a drop of a solution, one grain to a pint of water, or one of the atropized gelatines prepared for me by Savory and Moore, each of which contains one hundred thousandth of a grain of atropine, and will maintain dilation during several hours. This acts also perfectly well with rabbits or cats. [Illustration: Doctor examining patient.]