Collyria are divided by Dr. Jacob* into four classes, viz., Stimulant, Astringent, Sedative, and Escharotic.

1. Stimulants, as Vinum Opii, weak solutions of the Sulphates of Zinc and Copper, are most useful when there is a sensation of scalding, with a flow of tears, and intolerance of light; or when no other inconvenience remains, except a certain tenderness or rawness of the surface, depending, perhaps, upon a want of epithelium.

2. Astringents, As Alum, Tannin, Liq

Plumbi, are serviceable, both in acute and in chronic stages of Ophthalmia. Their mode of action is obscure: all that seems proved is, that the surface to which an astringent is applied becomes shrivelled and corrugated; but whether the vessels become diminished in calibre, or the secreting structures disabled from acting, or the epithelium coagulated by its operation, remains to be determined. Be the change, however, what it may, it seems to be proved by experience, that astringents diminish the enlargement of vessels caused by inflammation; in fact, remove the increased vascularity or redness, while they at the same time diminish or arrest the secretion of purulent matter, and perhaps benumb the sensitive villi or papillae.

3. Sedatives, as warm stupes, or tepid lotions, containing Opium, Henbane, Belladonna, &c. They are chiefly useful when the case assumes, a neuralgic character; but it appears undecided whether their beneficial effect is not derived as much from the warmth and moisture, as from the sedative properties of the medicines. The relief obtained from them is by no means constant. Vinum Opii seems to combine the properties of a sedative and stimulant, and is a peculiarly valuable application.

4. Escharotics, as saturated solutions of the Nitrate of Silver, the Sulphates of Copper and Alum, are useful in the acute and chronic stages of purulent Ophthalmia, in ulceration of the cornea, &c. They operate as chemical solvents of the tissues to which they are applied; and being so, must be very cautiously employed.

* Dub. Med. Press, May 7, 1S51.