en. LVIII.] 



THE PERIMETER 



839 



the crystalline lens of the eye E. They then again diverge and diffusely light up 

 the interior of the eyeball. The rays of light reflected from two points i and m on 



E 1 



IV! 



Fio. 532. The course of the light in examining the eye by the direct method. (T. G. Brodie.) 



the retina diverging from the eye are refracted by the glass lens L, and give an 

 inverted real image i 1 m l larger than the object i m. These latter rays then diverge, 



M 



Fio. 533. The course of the light in examining the eye by the indirect method. (T. G. Brodie.) 



and are collected and focussed by the observing eye E 1 to give an image i 2 m' 2 on the 

 retina. (T. G. Brodie.) 



The Perimeter. 



This is an instrument for mapping out the field of vision. It 

 consists of a graduated arc, which can be moved into any position, 

 and which when rotated traces out a hollow hemisphere. In the 

 centre of this the eye under examination is placed, the other eye 

 being closed. The examiner then determines on the surface of the 

 hemisphere those points at which the patient just ceases or just 

 begins to see a small object moved along the arc of the circle. These 

 points are plotted out on a chart graduated in degrees, and by con- 

 necting them the outline of the field of vision is obtained. 



Fig. 534 shows one of the forms of perimeter very generally 

 employed, and fig. 535 represents one of the charts provided with 



