ophthalmoscope. 331 



If the ophthaltnoscopic mirror without a lens gives an indis- 

 tinct 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, + i, + 2 or -f 3, dioptrics will 

 give the measure of the hypermetropia. 



If, on the contrary, the ophthalmoscopic mirror gives an indis- 

 tinct image of the fundus, which is rendered even more indis- 

 tinct by tiie 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, 

 — I dioptric, — 2 dioptrics, etc. 



The tendency in the horse is constanth' to slight long-sighted- 

 ness, 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 al.so 

 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 sub- 

 ject long sighted, during its action. 



Atropine tlie alkaloid of atropa belladonna is the most gener- 

 ally available and persistent of the mydriatics, and is in most com- 

 mon 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 ca.se of one attack the su.scepti- 

 bility 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 

 alkalinit}-, 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. 



