ABNORMALITIES IN THE REFRACTION OF THE EYE 



865 



axis, while the myopic and hypermetropic eyes refract them outward in an obUque 

 direction. Assuming then that the observed person is emmetropic and is accom- 

 modating for a far object, the parallel rays emitted by his retina must traverse the 

 central orifice in the mirror and be brought to a precise focus upon the retina of the 

 observer (L^). The latter thus obtains an erect niagnified image of the retina of the 

 observed person. A clear image, however, can only be obtained if both eyes are 

 emmetropic and are accommodated for the distance. Some difficulty may be 

 experienced at first in relaxing the accommodation, but this may be overcome if 

 one imagines himself gazing at an object placed far behind the eye of the patient. 

 A complete relaxation of the eye of the observed person is usually secured by the 

 administration of atropine, which alkaloid temporarily paralyzes the ciliary 

 mechanism. It also dilates the pupil, thereby preventing any interference on the 

 part of the iris with the reflection and refraction of the light. 



If the observed eye is myopic, ^ the rays of light emitted by the illuminated area 

 of its retina, are refracted into space as a convergent beam and cannot, therefore, 

 be focahzed by the emmetropic and relaxed eye of the observer (Fig. 465). In 



■■' P 



Fig, 464. — Direct Ophthalmoscopy. 

 Diagram to illustrate the remittance of the raj^s of light by an emmetropic eye. 

 O, observer's eye; M, mirror; P, patient's eye; F, the rays FA and FB, illuminate the 

 retina of P by a diffusion circle A^B^; L, the rays emitted by this luminous point are 

 brought to a precise focus in L^ of the observer's retina. 



order to bring these rays to a precise focus, they must first be rendered less conver- 

 gent by the interposition of a biconcave lens of sufficient diverging power to over- 

 come their excessive convergence. If the observed eye is hypermetropic (Fig. 

 466), the rays emitted by its illuminated retina, are divergent and cannot, there- 

 fore, enter the pupil of the observer. They may be made to do so, however, by 

 placing a biconvex lens in front of the orifice in the reflecting mirror. The strength 

 of the latter should be such that the formerly divergent rays now intersect in the 

 retina of the relaxed emmetropic eye of the observer. 



This method not only allows us to detect errors of refraction, but also to deter- 

 mine the strength of the lens which must be used by the patient in order to render 

 him emmetropic. Clearly, the strength and sign of the lens needed by him to 

 correct his defect, is indicated by the lens which the observer must employ in order 

 to obtain a clear image of his retina. For reasons stated previously, the weakest 

 concave lens should be prescribed for myopia and the strongest convex lens for 



1 If the observer moves his head and ophthalmoscope from side to side, the 

 retinal vessels will appear to move in the same direction in the hypermetropic and 

 in the opposite direction in the myopic eye. 



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