ELEMENTARY EXPERIMENTAL PHYSIOLOGY 261 



EXPERIMENT. Using Kuhne's artificial eye, place the retinal screen 

 in the position necessary to obtain a clear image of the external 

 luminous arrow. In this position of the retina the condition of the eye 

 may be regarded as emmetropic. Now move the screen about an inch 

 nearer the corneal surface. The image at once becomes blurred. This 

 represents the condition of the hypermetropic eye. Now place in front 

 of the cornea a very weak convex lens. The image will become much 

 clearer, and with little difficulty a lens of sufficient converging power 

 may be chosen which will exactly correct the defect. 



Myopia. This defect is usually congenital, the result of the antero 

 posterior diameter of the eye being too long. As a result parallel rays 

 are brought to a focus in front of the retina, and the eye cannot form a 

 clear image of an object beyond a certain distance (far point of 

 accommodation). The most common cause of acquired myopia in 

 children is the reading of books with insufficient light. The child 

 brings the book close to the eye to get a sufficiently large image of the 

 words and this finally leads to a myopic state. 



EXPERIMENT. Using again Kuhne's artificial eye, which, as in the 

 last experiment, is first adjusted as the emmetropic eye, shift the 

 retinal screen about an inch away from the cornea. The arrow now 

 becomes blurred and the eye resembles the myopic eye. Place in front 

 of the cornea a concave lens. The image will become much clearer if 

 the degree of concavity corresponds to that of the defect. It is neces- 

 sary in this case to use a lens of dispensive power in order that the 

 image may be thrown back on to the retina. 



CHAPTER LXXII. (Advanced). 

 THE OPTICAL DEFECTS OF THE EYE. 



1. Myopia and Hypermetropia. The condition of the refractive 

 media of the eye when either hypermetropia or myopia are present 

 are conveniently tested by what is known as the shadow test. If one 

 take a concave mirror (such as that of an ophthalmoscope used for 

 the indirect method), and reflects the light of a lamp at the side of 

 the subject into the pupil of the eye, on looking through the aperture 

 in the mirror the back of the eye is seen to be partially illuminated. 

 If the subject be emmetropic the amount of illumination is small, 

 and on tilting the mirror a little to the right or left a scarcely 

 perceptible movement of the light area may be seen in the opposite 

 direction of the tilt. The image of the lamp formed by the concave 



