DIOPTRICS OF THE EYE. 



327 



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 absorbed 

 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 hght near the eye does not, therefore, enable us to 

 see the interior more clearly, since in order to catch 

 the returning rays in our own eye it would be neces- 

 sary to interpose the head between the source of 

 light and the observed eye. If, however, we could 

 arrange the light to enter the observed eye as 

 though it proceeded from om- own eye, then the 

 returning rays would be perceived, and with suf- 

 ficient illumination the bottom or fundus of the 

 observed eye might be seen. Arguing in this way 

 Helmholtz constructed his first form of the oph- 

 thalmoscope in 1851. The value of the ophthal- 

 moscope is twofold: It enables the observer to 

 examine the interior of the eye and thus recognize 

 diseased conditions of the retina; it is also useful 

 in detecting abnormahties in the refractive sur- 

 faces of the eye. The principle of the instrument 

 is well represented in the original form devised by 

 Helmholtz, as shown schematically in Fig. 138, A. 

 I represents the observed eye and // the ej'e of the 

 observer. Between the two eyes is placed a piece 

 of glass inclined at an angle. Light from the can- 

 dle falling upon this glass is in part reflected from 

 the surface to enter eye /, and these rays on 

 emerging from the eye along the same line pass 

 through the glass in part and enter eye //. In 

 place of the plane unsilvered glass it is now cus- 

 tomary to use a concave mirror with a small hole 

 through the center, the observer's eye being placed 

 directly behind this hole. Such an instrument is 

 shown in Fig. 137. The instrument 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 

 hght into and out of the eye are represented 

 schematically in Fig. 138, B. The light from a 

 lamp or from an electrical bulb placed within the 

 handle of the ophthalmoscope (Fig. 137) is caught 

 upon the mirror and 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 hght. 



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 distance, 

 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- 

 laxing his accommodation, gaze through the eye, as it were, into the distance. 

 In this way he will see the portion of the retina illuminated, the image of the 



Fig. 137. — De Zeng electric 

 ophthalmoscope. The electric 

 light is contained in the handle 

 of the instrument and its light 

 is concentrated on the mirror 

 by a lens seen at the top of the 

 handle. Back of the mirror 

 is a rotating disc with plus 

 and minus lenses of different 

 powers. 



