306 



THE SPECIAL SENSES. 



ternary 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. 130. 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 light into and out of the eye are represented 

 schematically in Fig. 131,5. The light from a lamp caught upon the 



Fig. 131. 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 observer'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 rr,ys 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', &', c', 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. 



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 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 observer's 



