556 



PHYSIOLOGY 



metres in front of the observed eye (Fig. 28GB), the reflected rays issuing from the 

 pupil will be brought to a focus at a point between the observer and the lens, so that 

 at this point will be formed a real inverted image of the back of the eyeball. This 

 image, in the case of the normal eye, will lie at the focus of the lens. If the eye be 

 myopic, the convergent rays will be brought to a focus nearer to the lens than its 

 principal focus, while the divergent rays from the hypermetropic eye will give rise to 



an image in a plane between the principal 

 focus and the observer. From the figure 



(Fig. 281) it is evident that = , i.e. the 

 AB AO 



B 



FIG. 581. To illustrate how the rays 

 from an illuminated point of the retina 

 form a parallel beam on leaving the 

 eye, and are brought to a focus at B 

 by interposing the lens L. 



magnification of the image will be propor- 

 tional to the focal length of the lens used 

 divided by the posterior focal length of the 

 eyeball. If we are using a bi -convex lens of 

 10 cm. focal length and the eye be assumed 

 to have a posterior focal length of 1*5 cm., 

 the real inverted image that we see in the 



be , i.e. 6-7 times as 

 1-5 



bi-convex lens will 



large as the retinal structures represented. 



(6) THE DIRECT METHOD. In this method the observer places himself close 

 to the observed eye, throwing light into the latter from the mirror, and relaxes by 

 an effort of will his accommodation absolutely.* 



FIG. 282. 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. 



If both the observer's eye and the observed eye are normal and unaccommodated, 

 i.e. focused for distance, the rays of light, issuing from any point on the retina of the 

 observed eye, will leave the corneal surface as a beam of parallel rays, which on entering 

 the observing eye will in turn be focused to a point on its retina. The observer there- 

 fore sees an erect magnified image of the retina of the observed eye. If we take the 



* In the use of the opththalmoscope it is very difficult to relax accommodation 

 when trying to see something which is quite close. The student will find it an advan- 

 tage to try to imagine that he is looking through a telescope at an object at a con- 

 siderable distance off. He will then find the picture at the back of the eyeball s?uddc nly 

 come into view. 



