DIOPTRICS OF THE EYE. 



311 



in a purely objective way, so far as the patient is concerned. The most 

 important of these instruments are the ophthalmoscope, the retinoscope or 

 skiascope, and the ophthalmometer. A brief description is given of each 

 of these instruments, but for the numerous practical details necessary to 

 their successful use reference must be made to special manuals. 



The Ophthalmoscope. The light that falls into the eye is partly ab- 

 sorbed by the black pigment of the choroid coat and is partly reflected back 

 to the exterior. This latter portion is reflected back in the direction in which 

 it entered. Merely holding a light near 

 the eye does not, therefore, enatpe us to 

 see the interior more clearly, since in order 

 to catch the returning rays in our own 

 eye it would be necessary to interpose the 

 head between the source of light and the 

 observed eye. If, however, we could ar- 

 range the light to enter the observed eye 

 as though it proceeded from our own eye, 

 then the returning rays would be per- 

 ceived, and with sufficient illumination 

 the bottom or fundus of the observed eye 

 might be seen. Arguing in this way, 

 Helmholtz constructed his first form of 

 the ophthalmoscope in 1851. The value 

 of the ophthalmoscope is twofold: It en- 

 ables the observer to examine the interior 

 of the eye and thus recognize diseased 

 conditions of the retina; it is also useful 

 in detecting abnormalities in the refrac- 

 tive surfaces of the eye. The principle of 

 the instrument is well represented in the 

 original form devised by Helmholtz, as 

 shown schematically in Fig. 131, A. I 

 represents the observed eye and II the 

 eye of the observer. Between the two 

 eyes is placed a piece of glass inclined at 

 an angle. Light from the candle falling 

 upon this glass is in part reflected from 

 the surface to enter eye 7, and these rays 

 on emerging from the eye along the same 

 line pass through the glass in part and 

 enter eye II. In place of the plane un- 

 silvered glass it is now customary to use 

 a concave mirror with a small hole through 

 the center, the observer's eye being placed 

 directly behind this hole. Such an in- 

 strument is shown in Fig. 130. The in- 

 strument is used in two ways, known as 

 the direct and the indirect method. In the direct method the mirror is held 

 very close to the observed eye and the paths of the rays of light into and out 

 of the eye are represented schematically in Fig. 131, B. The light from a 

 lamp caught upon the mirror is thrown into the eye, the rays coming to a 

 focus and then spreading out so as to give a diffuse illumination of the fundus. 

 This latter surface may now be considered as a luminous object sending out 

 rays of light. Taking any three objects on the retina, A, B, C, it is apparent 

 that if eye 7 is an emmetropic eye these points are at the principal focal dis- 

 tance and the rays sent 'from each after emerging from the eye are in parallel 

 bundles. These rays penetrate the hole in the mirror and fall into the ob- 

 server's eye as though they came from distant objects. If the observer's eye 

 is also emmetropic, or is made so by suitable glasses, these bundles of rays 

 will be focused on his retina without an act of accommodation. He must, in 

 fact, in looking through the mirror, gaze, not at the eye before him, but, re- 



Fig. 130. Loring's ophthalmoscope. 



