620 



PHYSIOLOGY 



this method gives us a highly magnified image of the back of the eyeball, it is 

 of extreme value in judging of the existence of pathological conditions of the 



retina or choroid. It is also 

 of value in enabling the ocu- 

 list to determine by objective 

 methods the existence of any 

 errors of refraction in a 

 patient's eye. On examining 

 the eye by the direct method, 

 if the eye be myopic and the 

 rays leaving it convergent, it 

 will be impossible for the 

 observing eye to bring them 

 to a focus, and it will be 

 necessary to place a concave 

 lens in front of the hole in the 

 ophthalmoscopic mirror in 

 order to bring the back of 

 the observed eyeball into 

 view. The weakest divergent 

 lens through which an image 

 of the observed eye can be 

 obtained will give the degree 

 of myopia of the eye. On the 

 other hand, the rays from a 

 hypermetropic eye, being di- 



FIG. 280. 



Path of rays in examination by the 

 direct method. 



A, path of illuminating rays ; B, path of rays from 

 illuminated retina to observer's eye. 



vergent, will need a . certain 



effort of accommodation to bring them to a focus in the observer's eye, and here 

 the degree of hypernietropia will be given by the focus of the strongest 

 convex lens through 

 which it is just pos- 

 sible to obtain a clear 

 image of the retina 

 and retinal vessels. 

 By the same means 

 we may judge of the 

 existence of astigma- 

 tism and form an idea 

 of the meridians in 

 which the refractive 

 power of the eye is 

 faulty. For this pur- 

 pose observations are 

 taken of the focus of 

 the eye firstly, for 

 horizontal retinal 

 vessels ; secondly, for 

 vessels which are run- 

 ning vertically. p IG 



On examining the 

 back of the eyeball 

 by either of these 



methods, the most prominent object is the optic disc or optic nerve-papilla, 

 which marks the point of entrance of the optic nerve. It is seen as a pale oval 



281. Ophthalmoscopic view of fundus of eye, showing 

 the optic disc, or point of entry of the optic nerve, 

 with the retinal vessels branching from its centre. 



