OPHTHALMOSCOPE. 253 



Set up the model in a darkened room with the lens towards 

 you and on a level with your eye. 



Direct method of using the ophthalmoscope. First, with the 

 apertures closed, endeavour to look into the eye through the 

 lens, moving your eye and a light in all directions to do so. 

 You will not succeed. 



32Ctn | 



FIG. 52. E Eye model. L Light opposite the apertures in the funclus ; the 

 occluding card is not shown. * Lateral position of the light when the mirror M is 

 in use. A Amplifying lens. R Is placed in line with the retina of the observer's 

 eye. O Object, F and R (observer's retinal) real images. C Cerebral inversion of 

 the last. M Position of the mirror ; the central aperture of which is exaggerated. 



Secondly place the light behind the model, open the aper- 

 tures, and on looking into it you will see them and their 

 edges distinctly, the apertures being now radiant points of 

 light. Vision will be most distinct at a short distance from 

 the model. This corresponds to the direct method of using 

 the ophthalmoscope. 



Now close the apertures, place the light on one side of 

 the model and hold the mirror of the instrument in front of 

 and close to your own eye, look through its central aperture 

 and at a distance of about 7 cm direct light into the model. 

 The fundus and the points previously visible because they 

 emitted light are now seen because they reflect light in the 

 required direction. The mirror enables you to get over the 



