Before light can reach the retina, it has to pass through the cornea, which is covered by epithelium continuous with that of the conjunctiva. Alterations in either or both of these textures are therefore very important in regard to the integrity of vision. The chief drugs employed in the local treatment of diseases of the cornea and conjunctiva are warmth, moist and dry, anaesthetics, anodynes, antiphlogistics, antiseptics, and astringents. The chief astringents are perchloride of mercury, oxide of mercury, and nitrate of silver. The chief antiseptics are perchloride of mercury, quinine, boric acid, and sulphocarbolate of sodium. The chief sedatives are hydrocyanic acid, opium, belladonna, atropine, and cocaine. There are two astringents in common use which ought to be avoided, these are solutions of lead and of alum. Lead salts are objectionable, because if there is any ulceration on the cornea they may form an insoluble albuminate and cause permanent opacity. Salts of alum are said by Tweedy to be perhaps still more objectionable, because alum has the power of dissolving the cement by which the fibrillae of the cornea are held together, and this is very apt to give rise to perforation of the cornea whenever the epithelium is removed by injury or inflammation. Tweedy also thinks that strong solutions of common salt, ten per cent. or more, and solution of permanganate of potassium also dissolve the corneal cement and should therefore be avoided in inflammation of the conjunctiva or of the cornea. He considers that sulphate of zinc should be avoided, for the same reason, but it is largely used by others. The best astringent is probably perchloride of mercury, 1/64th to 1/16th of a grain to an ounce of water, and coloured with cochineal. The next best is an aqueous solution of boric acid, containing 3 to 8 grains of it with 3 to 10 grains of sulphocarbolate of sodium per ounce.

The chief effects which drugs produce on the eye, besides those just described, are alterations in the size of the pupil, in the power of accommodation, in the intra-ocular pressure, in the sensitiveness of the retina to impressions, and in the apparent colour of objects.