DIOPTRIC MECHANISMS OF THE EYEBALL 555 



When light falls on the eye through the pupil the greater part of it is absorbed 

 by the pigment of the retina and the choroid coat, A small amount, however, is 

 diffusely reflected, and is sent out by the way it came, viz. through the pupil. If the 

 vision is directed on the luminous point at the source of the rays, the reflected rays 

 leaving the eye will be converged to a point and form an image which will coincide 

 with the source of illumination. It is oft this account that the pupil always appears 

 black. When we look at a person's eye, we necessarily interpose our head and eye 

 between the observed eye and the source of light, so that no reflected light can come 

 back to our eyes. Only in albinos, where the pigment of the choroid coat and retina 



FIG. 280. Indirect ophthalmoscopy. 



A, course of rays from source of light E to observed eye ; o, observer's eye ; 

 M, mirror ; L, lens. 



B, course of rays from an illuminated spot on the retina of the observed eye to 

 the observer's eye. 



is lacking, do we get a red appearance, due to the reflected light passing through the 

 vascular tissues of the choroid and iris. 



In a hypermetropic eye at rest only those rays are brought to a focus on the retina 

 which are convergent as they enter the pupil. Light reflected from the retina of such 

 an eye will therefore be divergent as it leaves the pupil, and we may obtain a ' red 

 reflex ' by direct observation of the eye. 



In order that we may obtain an image of the interior of a normal eye we must 

 arrange that our eye coincides with the source of illumination. For this purpose 

 we use the device invented by Helmholtz, viz. a slightly concave mirror with a hole in 

 the centre. By means of this mirror light is converged on to the pupil, and the light 

 reflected by the retina is brought to a focus at the centre of the mirror, where is placed 

 the observer's eye. This ophthalmoscope may be used in one of two ways : 



(a) INDIRECT OPHTHALMOSCOPY. In carrying out ophthalmic observations 

 the examination is much facilitated by instilling atropine into the observed eye, so as to 

 dilate the pupil to the widest extent and paralyse the mechanism of accommodation . 

 If a beam of light be thrown into the pupil, the emergent rays from the eye will be parallel 

 and will give rise to a red reflection seen by the observer's eye at the centre of the ophthal- 

 moscopic 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 Jens of low power, say about ten dioptres (4 in. focus), be held a few centi- 



