618 



PHYSIOLOGY 



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 on 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 is lacking, do we get a red appear- 

 ance, due to the reflected light passing through the vascular tissues of the 

 choroid and iris. 



FIG. 278. Indirect ophthalraoscopy. 



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. 



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 observa- 

 tions 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 



