ABNORMALITIES IN THE REFRACTION OF THE EYE 863 



in order to subtend an angle of 60 seconds. Other letters may then 

 be constructed for the intervening distances by simply multiplying 

 the value of 0.2908 mm. by the distance (Fig. 462). This test, there- 

 fore, consists in determining the smallest retinal image, corresponding 

 to a visual angle of 60 seconds, which an eye is capable of perceiving. 

 If a person is unable to recognize this test type when held at its proper 

 distance, he is first made to look at it through a weak convex lens. If 

 this improves his vision, he is hypermetropic, because only an eye that 

 is too short or possesses a subnormal power of refraction, is in a position 

 properly to focalize convergent rays. He should be given the strong- 

 est convex lens with which he is able to see clearly, because clear vision 

 then forces him to relax his accommodation as much as possible. If, on 

 the other hand, the vision of the patient is more highly impaired by 

 the interposition of convex glasses, he is myopic and requires spectacles 

 with concave lenses. In this case, the lenses prescribed for him, should 

 be the weakest with which he is still able to see clearly, because this 

 forces him to bring his ciliary mechanism into physiological play. It 

 is evident that this test should also be made separately for each eye. 

 Instead of the test letters, ordinary print held at the proper reading 

 distance, may be used. 



The Ophthahnoscopic Method. — The eye is a camera obscura, and 

 its interior is not open to direct inspection, because the choroid and iris 

 are pigmented and practically impermeable to light. Even in the 

 albino, nothing more than a slight ''reflex" sensation of pink is ob- 

 tained. The fundus of the eye also remains absolutely invisible if we 

 look through the pupillar orifice, because we must then assume a posi- 

 tion directly in -front of the head of the observed person. Obviously, 

 the rays of light are thereby prevented from entering the vitreous 

 chamber. In some animals, however, the visual axes are more diver- 

 gent so that the rays can get past the observer's head to illtiminate the 

 retina. 



Whenever light is reflected into an eye, a large part of it is absorbed 

 by the pigment of the choroid, while a small portion of it is refracted 

 outward into space in the same direction in which it entered. It 

 must be evident that if a luminous point in space L is accurately cen- 

 tered upon the retina in L', this focal point L' remits divergent rays 

 which are again rendered convergent by the lens to be intersected in L, 

 Consequently, L and U are conjugate foci. This outward refraction 

 is made impossible if we adjust our eyes to look into the pupillar orifice 

 of the patient, because we thereby cut off the supply of light rays and 

 render the retina non-luminous. In 1851 Helmholtz conceived the 

 idea of illuminating the eye from a lateral source of light by means of 

 three mirrors placed at an angle of 56° to the line of light. This instru- 

 ment which he called the ophthalmoscope (Fig. 463), has been modified 

 repeatedly, but the principle involved in its construction has remained 

 the same. In its modern form it consists of a concave silvered mirror 

 by means of which light is reflected into the patient's eye from a gas- 



