DIOPTRICS OF THE EYE. 307 



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 accom- 

 modation. He must, in fact, in looking through the mirror gaze, 

 not at the eye before him, but, relaxing 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 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 principal focus of its refracting surfaces; consequently 

 the rays sent out from the illuminated retina emerge in converging 

 bundles and can not 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 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 ob- 

 server unless he exerts his own power of accommodation or inter- 

 poses a convex lens between his eye and the mirror. 



The indirect method of using the ophthalmoscope is represented 

 in Fig. 131, C. The mirror is held at some distance, at arm's length, 

 from the observed eye, /, 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 in the eye, whence they again diverge and light up 

 the retina with a diffuse illumination. The light from this retina 

 is in turn sent back toward the mirror, its path being indicated for 

 the point b by the black lines. If the eye is emmetropic the rays 

 from this point emerge parallel, and falling upon the biconvex lens 

 are brought to a focus at b'. Similarly the rays from a will be 

 brought to a focus at a' and from c at c'. Consequently there will 

 be formed in the air an inverted image, and it is at this image that 

 the eye of the observer gazes through the hole in the mirror. This 

 image forms its image on the retina of the observer's eye, as repre- 

 sented in the diagram at a", b", c", and is projected outward or seen 

 inverted as regards the original position of the points in the retina 

 of eye /. The indirect method is the one usually employed in 

 ophthalmoscopic examinations of the retina. It gives a larger 

 field than the direct method, although the objects seen are of 

 smaller size. 



