ILLUMINATION OF THE EYE, AND THE OPHTHALMOSCOPE. 



8 49 



same dioptric law, an enlarged, inverted real image (c d) must be formed 

 outside of the eye (at A B) of any illuminated, circumscribed portion of 

 the retina (when the eye is accommodated for a certain distance). If 

 the eyeground is sufficiently illuminated, this image formed in the air 

 will possess a corresponding degree of brightness. 



In order to examine more carefully the individual portions of this 

 image of the retina, the observer must accommodate for the situation 

 of the image. His eye is then separated from the retina of the eye under 

 observation a distance equal to the sum of the focal distances of his own 

 and the other eye. At this distance the finer details of the eyeground 

 cannot be recognized. Moreover, as the pupil of the eye examined is 

 narrow, only a small portion of the eyeground can be seen, and only 

 under a low visual angle; aside from this, it is often impossible to 

 accommodate for the image. 



It is necessary, therefore, to bring the eye of the observer closer 

 to the eye under examina- 

 tion. This may be done in 

 two ways, (i) By placing 

 before the eye under exam- 

 ination a strong convex lens 

 with a focus of i inch (Fig. 



293, C). As the image of the 

 retina is thus brought closer 

 to the eye (at B), as the result 

 of the refraction of the rays 

 by the lens, the observer M 

 can approach much nearer 

 and can still accommodate 

 for the image. (2) Or by 

 placing a concave lens (Fig. 



294, o) before the eye exam- 

 ined. The rays emerging 

 from this eye are either made 

 parallel by the concave lens 

 o, and are then focused on 

 the retina of the emmetropic 

 observer A; or, if the lens 

 make the rays divergent 

 (Fig. 295), an upright, virtual 

 image of the eyeground is 



formed in the distance, behind the investigated eye (at R). Under 

 such circumstances also the observer can approach much nearer, 



The illuminating apparatus, together with one of these lenses forms 

 the ophthalmoscope of v. Helmholtz, the basis of modern ophthalmo- 

 scopy, by means of which all the details of the eyeground can be 

 examined. 



For the illumination, v. Helmholtz used several plates of glass placed behind 

 one another (for better reflection), in the same position as 55 in Fig. 292. A 

 plane mirror or a concave mirror, with a focus of 7 inches, through the center of 

 which a hole is bored (Fig. 293, Si, 5 2 ) may also be employed. Fig. 296 shows 

 the ophthalmoscopic appearance of the optic-nerve entrance, and its vicinity, 

 in a normal eye. The letters indicate the details. In albinos the fundus appears 

 light red, because light passes into the eye through the nonpigmented sclera 



54 



FIG. 296. The Optic-nerve Entrance, with the Surrounding 

 Structures, of a Normal Eyeground (after Ed. Jaeger): 

 A, Optic disc (papilla); a, connective tissue ring ; b, cho- 

 roidal ring; c, arteries; d, veins; g, point of division of the 

 central artery; h, of the central vein; L, lamina cribrosa; 

 /, temporal (outer) side; n, nasal (inner) side. 



