LXXV.] OPHTHALMOSCOPE. 365 



The observer places liimself ahout 20 to 18 inches from the patient, 

 and, holding the mirror in his right hand, by means of it throws 

 a beam of light into the eye of the patient. When the eye is 

 illuminated, he takes a small biconvex lens of 2 to 3 inches focus 

 in his unemployed hand — the left in this case — holding it between 

 his thumb and index-finger, placing it vertically 2 or 3 inches from 

 the observed eye. To ensure that the lens is held steadily, rest the 

 little finger upon the temple or forehead of the patient. Keep the 

 lens steady, and move the mirror until the optic disc is seen, with 

 the details already described. 



In the direct method only a small part of the retina is seen at 

 one time, but it is considerably magnified ; while by the indirect 

 method, although more of the retina is seen at once, it is magni- 

 fied only sHghtly. 



If the observed or observer's eye is abnormal, suitable glasses to 

 be fixed behind the mirror are supplied with every ophthalmoscope. 

 In some forms of ophthalmoscope, /» 



such as that of Gowers and others, If' ^_:=^-===i\ 



these lenses (convex + , and con- ^iLNr"'^"^' - ^ v I \v 



cave - ) are fixed to a rotating xvWiP' --^-^^ 



disc behind the mirror. As the ^-^Pl', 



disc is rotated, lens after lens can ^ /^'"^^i^^^illillllilliilili^ 

 be brought to He exactly behind '»i2-'^il|l^ i 

 the hole in the mirror, and thus ^^Bffl I 1 1111111 

 correct any anomaly of refraction. ^ffl||ip||^^ 



3. Eye of a Living Rabbit. Fiq. 28 5, -carriage for Rabbit. 



Instil atropine as before, or 

 use an atropinised gelatine disc to effect the same result. Place 

 the rabbit in a suitable cage to keep it from moving. A suitable 

 one was devised by ^Michel ; use it (fig. 285). Examine the eye by 

 the direct and indirect methods. N.B. — If an albino rabbit be 

 used the observer sees the large choroidal vessels. 



4. Perrin's Artificial Eye. 



Use this until a clear image of the fundus is obtained by both methods. 

 In fact, it is well for the student to begin with this. In this model, eye-caps 

 to fit on to the eye are supplied, so as to render the eye-model eitlier myopic 

 or hypermetropic. Afterwards test these, and use the necessary lenses behind 

 the mirror to correct these errors in the shape of the eyeball. 



Frost's artificial eye, as made by Curry and Paxtou, is also useful, as is also 

 that of Priestley Smith. 



5. Kiihne's Method. — If an artificial eye is not at hand, a very 

 suitable arrangement is that devised by Kiihne. Paint a disc to 

 resemble the normal fundus when it is seen with the ophthalmO' 



