332 



THE SPECIAL SENSES. 



objects seen being inverted on his own retina and therefore projected or seen 

 erect. If the observed eye is myopic its retina is farther back than the prin- 

 cipal focus of its refracting surfaces; consequently the rays sent out from the 

 illuminated retina emerge in converging bundles and cannot be focused on 

 the retina of the observer's eye. By inserting a concave lens of proper power 

 between his eye and the mirror the observer can render the rays parallel and 

 thus bring out the image. From the power of the lens used the degree of my- 

 opia may be estimated. Just the reverse happens if the observed eye is 

 hypermetropic. In such an eye the retina is nearer than the principal focal 



Fig. 138. Diagrams to represent the principle of the ophthalmoscope : A, The orig- 

 inal form of ophthalmoscope, consisting of a piece of glass, M, inclined at a suitable angle. 

 The rays from the source of light are reflected into the observed eye, /, and thence return 

 along the same lines passing through M to reach the 9bserver's eye, //. B, the direct 

 method with the ophthalmoscopic mirror. The rays of light illuminate the fundus of the 

 observed eye, /, and thence pass out in parallel rays, if the eye is emmetropic, to reach the 

 observer's eye, //. C, the indirect method with ophthalmoscopic mirror and intercalated 

 lens. The rays of light-red lines are brought to a focus within the anterior chamber of the. 

 eye and thence diverge to give a general illumination of the interior of the eyeball. The 

 returning rays of light are indicated for a single point, 6. At a', 6', cf ', a, real inverted image 

 of a portion of the retina is formed in the air, which in turn is focused on the retina of the 

 observer's eye. 



distance of the refractive surface; consequently the light emitted from the 

 retina emerges in bundles of diverging rays which cannot be brought to a 

 focus on the retina of the observer unless he exerts his own power of accom- 

 modation or interposes a convex lens between his eye and the mirror. 



The indirect method of using the ophthalmoscope is represented in Fig. 

 138, C. The mirror is held at some distance, at arm's length, from the ob- 

 served eye, 7, while just before this eye a biconvex lens of short focus is 

 placed. As shown in the diagram by the red lines, the reflected light from 

 the mirror comes to a focus and then diverging falls upon the biconvex lens. 

 This lens brings the rays to a focus at or near the eye, whence they again 

 diverge and light up the retina with a diffuse illumination. The light from 



