ELEMENTARY EXPERIMENTAL PHYSIOLOGY 115 



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 XXIX. 



THE INSTRUMENTS USED IN THE CLINICAL INVESTIGATION OF 



THE EYE. 



The Measurement of the Field of Vision.- If the eye be fixedly 

 directed to some particular point it is possible to see objects at some 

 distance from this point. The area in which objects can be seen with the 

 eye thus fixated is spoken of as the field of vision. With the head fixed 

 and the eye allowed to move as far as possible in any direction a much 

 larger area can be viewed. This area is spoken of as the field of regard. 



Though fairly satisfactory results can be obtained by using a com- 

 paratively simple form of apparatus called a campimeter, it is customary 

 to employ an instrument called a perimeter to obtain accurate details of 

 the extent of the field of vision. 



The perimeter (see Fig. 97), consists of a quadrant upon which a 

 white spot can be moved, and this quadrant can be revolved about a line 

 continuous with the optic axis. At K is the chin rest, double, so as to 

 enable either eye to be adjusted against 0. The subject having taken 



