620 



PHYSIOLOGY 



FIG. 280. 



Path of rays in examination by the 

 , direct method. 



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 



A, path of illuminating rays ; B, path of rays from other hand, the rays from a 

 illuminated retina to observer's eye. hypermetropic eye, being di- 



vergent, will need a certain 



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



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



convex lens through 



which it is just pos- 



sibte 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. FlG 2 81. Ophthalmoscopic view of fundus of eye, showing 

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



back of the eyeball with the retinal vessels branching from its centre. 



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 



