766 



THE OPHTHALMOSCOPE. 



the eye of the patient by his own visual distance, and by the visual distance of the patient. As 

 this distance is considerable, the individual small details of the fundus cannot be seen distinctly. 

 Further, owing to the contraction of the pupil of the patient, only a small area of the fundus 

 can be seen, and this only under a small visual angle, quite apart from the fact that it is often 

 impossible to accommodate for the real image of the fundus of the patient. 



Hence, the eye of the observer must be brought nearer to the eye of the patient. This may 

 be done in two ways : (1) Either by placing in front of the eye of the patient a strong convex 

 lens (of 1 to 3 inches focus fig. 549, C). This causes the retinal image to be nearer to the eye 

 (at B), owing to the strong lens refracting the rays of light The observer (M) can come nearer 

 to the eye, and can still accommodate for the image of the fundus of the eye. (2) Or a concave 

 lens is placed immediately in front of the eye of the patient (fig. 550, o). The rays of light 

 emerging from the eye of the patient (P) are either made parallel by the concave lens (o), and 

 are brought to a focus on the retina of the emmeti-opic observer (A) ; or, if the lens causes the 

 rays to diverge (fig. 551), an erect, virtual image is formed at a distance behind the eye of the 

 patient (at R). In these cases, also, the observer can go much nearer to the eye of the patient. 



The ophthalmoscope invented by v. Helmholtz enables us to examine the whole 

 of the fundus of the eye. 



[Direct Method. Use a concave mirror of 20 centimetres focal distance, with a central open- 

 ing. Reflect a beam of light into the patient's eye, where the rays cross in the vitreous and 



Fig. 553. 



Fig. 552. -The entrance of the optic nerve with the adjacent parts of the fundus of the normal 

 eve. a, ring of connective-tissue ; b, choroidal ring ; c, arteries ; d, veins ; g, division of 

 the central artery; h division of the central vein ; L, lamina cribrosa : t, temporal (outer) 

 side ; n, nasal (inner) side. Fig. 553. Morton's ophthalmoscope. 



illuminate the fundus of the eye. These rays again pass out of the eye and reach the observer's 

 eye through the central hole in the mirror. If the observer be emmetropic they come to a focus 

 on his retina. In this way all the parts of the retina are seen in their normal position, but 

 enlarged. Hence it is sometimes called the examination of the upright image. The eye of 

 the patient and observer must be at rest, i.e., be negatively accommodated, while the mirror 

 must be brought as near as possible to the eye of the patient] 



[indirect Method, by which a more general view of the fundus is obtained. Throw the light 

 into the patient s eye by an ophthalmoscopic mirror as above, but held ata distance of about 50 

 cm (10 inches) from the patient's eye. Hold a biconvex lens of 14 dioptrics focal length 

 vertical y Wtween the mirror and the patient's eye (fig. 549), the observer looking through the 

 hole of the mirror. What he does see is an inverted aerial image at B. Only a small part of 

 the fundus ocuh can be seen at one time.] 



[The ophthalmoscope, besides being used for examining the interior of the eyeball, is of the 

 Utmost use in determining the existence and amount of anomaUes of refraction in the refrac- 

 UnL v i. For u 1S PU ,T S u an P h , tha lscope requires to be provided with plus and minus 

 In Inln? ^ , y *? ht **> the eye of the observer. This is readily done by 

 "'fj " """J cha " 18m ^r 1 ^ C i 0Ur)er ' and made use of in the hand y students' ophthal- 

 moscope of Morton (hg. 553). The lenses are moved by a driving-wheel on the left figure, 



