THE OPTICAL DEFECTS OF THE EYK. 19 



4. Myopia can be distinguished from every other defect of vision, 

 by the fact that concave glasses improve vision for distant objects. 

 If "we have no concave glasses convenient, we can diagnose it from 

 Amphyopia, (insensibility of the retina) by the following ready 

 method : — A person with normal vision can read distinctly, No. I 

 test type at 12 inches, and even a little farther. We will suppose 

 that a patient's vision is so impaired, that he can only read No. II 

 at 6 inches ; if he is not also myopic, he can also read No. IV at 12 

 inches, or No. LX at 180 inches — that is at 15 feet. However im- 

 paired then a person's vision may be, unless he be also myopic, he' 

 can see as well proportionately, at one distance as at another. On 

 the contrary, a person with myopi.'i, say i, can see the smallest type 

 (much smaller than No. I,) at 6 inches, but he cannot see No. IT, or 

 even No. V, at 12 inches. 



This disease is often herediiary, Over exertion of the eyes upon 

 near objects at the age of puberty, (about 14 or 15) is a very fre- 

 quent cause of myopia. 



Short-sighted persons often inquire if we would advise the use 

 of spectacles. There can be no objection to wearing glasses that 

 will enable them to see distant objects ; for their eyes are thus 

 changed to normal ones, but as most persons use their eyes much 

 more frequently upon near than upon distant objects ; the glasses 

 should be no stronger than necessary. Some contend, however^ 

 that short-sighted persons should dispense with glasses for reading,, 

 writing, &c. Prof. Douders, however, recommends their use for this 

 purpose, for the following reasons : — 



Ist. " Because strong convergence of the optic axes is necessarily 

 paired with tension of the accommodation. The latter is an associ- 

 ated action, not arising from the mechanism of the convergence, but? 

 existing within the eye itself, and may consequently easily lead to an 

 increase of the myopia. Besides this, the pressure of the muscles 

 upon the eye ball appears to be greater when the optic axes are 

 convergent, than when they are parallel, and this increase of pres- 

 sure cannot but tend to give rise to the development of posterior 

 staphyloma. 



2d. " On account of tho habit which short-sighted persons have of 

 bending their head forwards during reading or writing. This must 

 cause an increased flow of blood to the eye, and an increased tension 

 within the eye itself. Owing to this development of sclerotico — cho- 

 roiditis posterior, effusions of blood and detachment of the retina. 



