24 THE OPTICAL DEFECTS OF THE EYE. 



Had bis eye been bypermetropic, parallel rays would not bave been 

 sufficiently converged by tbe refractive power of the eye, to form a 

 focus upon tbe retina ; vision would, consequently, bave been indis- 

 tinct. By placing, however, tbe proper convex lens in frqjiit of such 

 an eye, tbe requisite preliminary convergence would be given to tbe 

 rays, to enable tbe eye, with its low refractive power, to focus these 

 rays upon tbe retina, and thus render vision distinct. 



Tbe lens used in such a case would indicate the degree of byper- 

 metropia. If tbe lens w^ere a + 15 inch, the hypermetropia would 

 equal -^V if a + 10, tbe hypermetropia would be -^f and so forth, 



"We bave, however, tbe means of temporarily producing this con- 

 dition of tbe eye by artificial means. By applying a four grain 

 solution of atropine to the eye, within two hours tbe action of tbe 

 ciliary muscle will be completely ptaralysed. A solution of one grain 

 of atropine to an ounce of pure water (also a solution of tbe extract 

 of belladonna) will dilate the pupil widely, and in some cases, will 

 render the eye slightly presbyopic, but it will not paralyse tbe accom- 

 modation. 



If we test, in this manner, the case of suspected hypermetropia 

 mentioned above, and find that after bis accommodation is para- 

 lysed, be is not able to read No. xxx. even with -f- 50, and that 

 tbe only glass with which be can read No. xv. and No. xx. at 

 20 feet is 4- 20 ; his hypermetropia is therefore -^. But as be 

 could see as well with + 50 as without them, before bis accom- 

 modation was paralysed ; be bad a manifest hypermetropia of ■^. 

 Tbe difference between his total hypermetropia and his manifest 

 hypermetropia will give tbe amount of tbe latent hypermetropia, 

 whicb he overcame with tbe exercise of bis accommodation, namely, 



1 flino 1 1 — 1 * 



■33-J, lUUS ^-^ jQ -g-gj. 



Asthenopia, according to Bonders, depends almost invariably on 

 hypermetropia. He describes it as follows : " The power of vision is 

 usually acute, — and nevertheless, in reading, writing, and other close 

 work, especially by artificial light, or in a gloomy place, the objects 

 after a short time, become indistinct and confused, and a feeling of 

 fatigue and tension comes on in, and especially above the eyes, neces- 

 sitating a suspension of work. The person affected now often invol- 

 untarily closes bis eyes, and rubs his hand over tbe forehead and 



• Hypermetropia can casilj be diagnosed with the ophthalmoscope. 



