This is so essential to clear and definite conclusions and is so easily practiced on the domestic animals that every veterinarian should make himself familiar with the method. The method is based on the fact that when two perfectly transparent media touch each other a reflection of luminous rays takes place only at the surface. But in case any opacity exists in any part of the thickness of one of these media, it reflects the rays from its surface no matter what may be its position in the medium. Thus corneal opacities appear as gray blotches and under careful focal illumination it may be determined whether these are on the conjunctival surface, in the superficial or deeper layers of the cornea or in the membrane of Descemet. Similarly cloudiness or floating objects in the aqueous, reflect the luminous rays, and so with opacities in the lens or its capsule, or in the vitreous. In the same way the surface of the iris and corpora nigra may be carefully scrutinized. For satisfactory examination of the media, back of the iris, the pupil should be first dilated, by instillation under the lid of a drop or two of a 3 per cent. solution of atropia, and the examination proceeded with twenty minutes later. Homatropin is preferable to atropin as being less persistent in its action, and less liable to produce conjunctivitis. If it fails to produce the requisite dilatation, it may be followed by a drop of a 4 per cent. solution of hydrochloride of cocaine, which will secure a free dilatation, lasting only for one day in place of seven days as with atropin. The cocaine further removes pain and favors the full eversion of the eyelids. The instruments required for focal illumination are a biconvex lens of 15 to 20 diopters, and a good oil lamp or movable gas jet. The light of the sun is not satisfactory. The examination ought to be conducted in a dark room, or less satisfactorily in semi-darkness. The lamp is held by an assistant at the level of the eye to be examined, either in front or behind, or first one and then the other, so that the rays of light may fall upon the eye obliquely. If the lids are kept closed it may be necessary to expose the cornea by pressing on the lids with the finger and thumb. The light is held 8 or 10 inches from the eye and the lens is interposed between it and the eye and moved nearer and more distant until the clearest illumination has been obtained of the point to be examined. In this way every accessible part of the eye may be examined in turn. The examiner may make his results more satisfactory by observing the illuminated surface through a lens magnifying three or four diameters. It is important to observe that the eye of the operator must be in the direct line of reflection of the pencil of light.