354 PHYSIOLOGICAL PHYSICS. [Chap, xxvin. 



A converging lens is also frequently employed. 

 The effect on the reflected rays is shown in Fig. 160. 

 The observed eye is represented by A, and a is a point 

 on the retina. The reflected rays passing through the 

 media of the eye would, if permitted, meet at 6, and an 

 image of part of the retina represented by the arrow 

 a would be produced, represented by the arrow eb. 

 1 But the convex lens B refracts the rays still more, 

 and the result is that the rays are focussed at d, 

 nearer to the observed eye than eb. Consequently the 



n- 



Fig. 160. The Ophthalmoscope, with Inverted Image. 



observer's eye c, placed at the distance for distinct 

 vision, sees an image f, smaller than eb, inverted, and 

 real. 



Thus the ophthalmoscopic mirror, alone or in con- 

 junction with a concave lens, gives a virtual erect 

 image, considerably magnified. The convex lens gives 

 a real inverted image, and considerably less magnified. 

 The focal length of the convex lens usually employed 

 is about 6 centimetres. With such a lens the extent 

 of magnification is about four times, but with a lens 

 of longer focus it would be increased. 



There are many forms of ophthalmoscope which it 

 is not necessary to consider here. In general the 

 concave mirror ought to have a focal length of about 



* ' O 



18 centimetres, and the convex lens one of 6 cc. 

 Usually there are also supplied two discs, each of 

 which has, round its circumference, a series of circular j 

 openings, about 8 millimetres in diameter. In one 

 disc, each of these openings is occupied by small con- 

 vex lenses of varying focal length, in the other by 



