222 EXPERIMENTAL PHYSIOLOGY 



refracted than in the previous case. The rays from k meet accurately 

 upon the retina at r, and produce a clear image there. The rays from 

 p are more refracted, and meet at a point in front of the retina, cross, 

 and impinge at two positions, p x and p 2 , upon the retina, where they 

 give rise to the sensation of two images of the needle P indistinct and 

 blurred. The image j? will be referred as coming from an object 

 lying to the right of that caused by p l . On now blocking the left- 

 hand pin-hole a the image formed by p l disappears. This is the 

 left-hand image. Similarly on blocking the right-hand hole b the 

 right-hand image due to p 2 disappears. 



The ophthalmoscope. — On looking at an eye the pupil always 

 appears black. This is because most of the rays entering the eye 

 are absorbed by the retinal and choroidal pigment, and those few 

 which are reflected travel back along nearly the same path as that 

 they took on entering the eye. To see any part of the interior of the 

 eye, these reflected rays must enter the observing eye ; but as soon as 

 the eye is placed to intercept them, it also blocks the course of the 

 entering light. The first condition, then, is to be able to receive the 

 reflected rays without at the same time intercepting the rays from the 

 source of light. 



This is attained by the instrument invented by von Hehnholtz, 

 the ophthalmoscope. The principle of this is to reflect light into the 

 eye by a mirror in the centre of which is a small aperture through 

 which the observer looks, and is thus able to receive some of the rays 

 reflected from the interior of the observed eye. 



There are two methods of employing the ophthalmoscope, which are 

 known as the indirect and direct methods respectively. 



I. The indirect method. — The person whose eye is to be examined is 

 seated in a darkened room with a large steady flame placed a few inches 

 from his head on his left side on a level with his eyes. For examining his 

 right eye take the ophthalmoscope mirror in your right hand, and with the 

 mirrortowards the observed eye look through the central aperture, with the 

 right eye so that (with the left eye closed) you can see the observed eye 

 clearly. Now open the left eye, and watching the position of the reflected 

 light rotate the mirror until the reflected light is thrown on to the eye. Now 

 tell him to look steadily at some object behind you at the other end of the 

 room. The pupil will now become a bright red. Get him to move his eye 

 in various directions and in one position, when it is turned a little inwards ; 

 the red will change to a yellowish colour. This indicates the position of the 

 optic disc. Now take the large biconvex lens in the left hand, holding it 

 vertically about 2 to 3 inches from the eye, steadying your hand by resting 

 your little finger on the temple. Your eye shoidd be about 15 inches from his. 

 In this position an image of the fundus of the eye will be formed by the lens 

 about 2 or 3 inches in front of it, i.e. about 10 inches from your eye, and you 

 will be able to see this image, which if the observed eye have not been moved 

 will be one of the optic disc. Most beginners find some difficulty in avoiding 

 the reflection from the cornea and in adjusting the accommodation, and the 

 distance of the head, so as to see the image clearly. The head must be 



