DIOPTRIC MECHANISMS OF THE EYEBALL 619 



the observer's eye at the centre of the ophthalmoscopic mirror. If the eye 

 be myopic, the issuing rays will be convergent and will therefore be brought 

 to a focus at some point in front of the eye, giving rise to a real image 

 of the retina. If the eye be hypermetropic, the issuing rays will be 

 divergent, and the observer will see the red reflection of light from the back 

 of the retina. 



If now a lens of low power, say about ten dioptres (4 in. focus), be held a few 

 centimetres in front of the observed eye (Fig. 278s), 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 



. A 



FIG. 279. 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. 



principal focus and the observer. From the figure (Fig. 279) it is evident 



t h at -& = ^_, i. e . the magnification of the image will be proportional to 



AB AO 



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 %ye 

 be assumed to have a posterior focal length of l-5cm., the real inverted image 



that we see in front of the bi-convex lens will be , i.e. 6-7 times as large as 



the retinal structures represented. 



(&) 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.* 



If both the observer's eye and the observed eye are normal and unaccom- 

 modated, 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 therefore sees an erect magnified image 

 of the retina of the observed eye. If we take the focus of the eye as 1-5 cm. 

 the magnification of the image is equivalent to that which would be produced 



20 

 by a lens of the same focus and is equal to y^, i.e. about thirteen times. Since 



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

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

 an advantage to try to imagine that he is looking through a telescope at an 

 object at a considerable distance off. He will then find the picture at the back 

 of the eyeball suddenly come into view. 



