DIOPTRIC MECHANISMS OF THE EYEBALL 557 



focus of the eye as 1-5 cm. the magnification of the image is equivalent to that which 



OA 



would be produced by a lens of the same focus and is equal to , i.e. about thirteen times. 



1*0 



Since 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 oculist 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 con- 

 vergent, it will be impossible for the 

 observing eye to bring them to a <* ^ 



focus, and it will be necessary to v,i 



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 hypermetro- 

 pic eye, being divergent, 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 possible 

 to obtain a clear image of the retina 

 and retinal vessels. By the same 

 means we may judge of the existence 



of astigmatism and form an idea of the meridians in which the refractive power of the 

 eye is faulty. For this purpose observations are taken of the focus of the eye firstly, 

 for horizontal retinal vessels ; secondly, for vessels which are running vertically. 



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 disc surrounded by a deep red background (Fig. 

 283). From the middle of the papilla the retinal vessels pass into the eyeball, and 

 they are seen diverging from the papilla to ramify over the rest of the retina. The 

 arteries can be distinguished from the veins by their brighter red colour as well as by the 

 stronger reflection of light from their surfaces. The yellow spot is very difficult to see, 

 except in atropinised eyes, since it only comes into view when the observed eye is looking 

 straight into the ophthalmoscope. Under these conditions there is a strong ' light 

 reflex,' and the pupil contracts up to a pin-point, unless paralysed by means of atropine. 

 In order to see the blind-spot, or optic disc, the observed eye must be directed inwards ; 

 thus if A is looking at the right eye of B, B must be told to look over A's right 

 shoulder. 



FIG. 283. 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. 



