This can be best done in the lower animals by determining the strength of the lens required to render clear the image of its fundus. By knowing the refracting power of the lens, we may ascertain what deviation from the normal refraction there is in the eye under observation. In making this test the mirror of the ophthalmoscope must be brought closely to the eye of the patient—1 to 2 inches. If in such a case and without the use of any lens a distinct image of the fundus is obtained, and if this is rendered less distinct by interposing the lowest convex lens in front of the eye of the observer, the eye is =emmetropic=. If the ophthalmoscopic mirror without a lens gives an indistinct vision of the fundus, and if the image is rendered clear by interposing one of the convex lenses, the eye is =hypermetropic=. The strength of the convex lens, +1, +2 or +3, dioptrics will give the measure of the hypermetropia. If, on the contrary, the ophthalmoscopic mirror gives an indistinct image of the fundus, which is rendered even more indistinct by the interposition of a convex lens, but is cleared up and rendered definite by a concave lens, the patient is myopic. The strength of the concave lens used will give the degree of _myopia_, –1 dioptric, –2 dioptrics, etc. The tendency in the horse is constantly to slight long-sightedness, but the deviation is rarely found to be serious either in this direction or in that of astigmatism. Mydriatics. Dilation of the pupil by mydriatics (mydriasis dilation of the pupil) is a most important means of diagnosis, and therefore a knowledge of the action of the different mydriatics is essential. The mydriatics in common use not only dilate the pupil, but also paralyze the ciliary body and the power of accommodation in ratio with the strength of the solution employed. This determines an adaptation of the eye to the farthest point of vision and holds it there until the action of the mydriatic passes off and normal power of accommodation is restored. In short it renders the subject long sighted, during its action. =Atropine= the alkaloid of atropa belladonna is the most generally available and persistent of the mydriatics, and is in most common use. It is usually employed as sulphate of atropine, though some prefer the nitrate, the salicylate or the borate to obviate the danger of atropinism. This form of poisoning may show in the occurrence of conjunctivitis and in case of one attack the susceptibility to atropine is greatly to be dreaded, so that it should never again be used on the same subject. The real cause of atropinism is uncertain, it has been variously ascribed to too great acidity or alkalinity, or to micro-organisms growing in the solution. Hence the importance of using the antiseptic salts of atropine, and of testing the solution to see that it is exactly neutral before it is applied.