CM. I.VI.] THE PEKIMETEH 795 



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

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

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

 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, 



FIG. 591. Priestley Smith's Perimeter. 



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. 591 shows one of the forms of perimeter very generally 

 employed, and fig. 592 represents one of the charts provided with 

 the instrument. The blind spot is shown, and the dotted line 

 represents the normal average field of vision for the right eye. 



It will be seen that the field of vision is most extensive on the 

 outer side ; it is less on the inner side because of the presence of the 

 nose. 



By the use of the same instrument, it is found that the colour 

 of a coloured object is not distinguishable at the margin, but only 

 towards the centre of the field of vision, but there are differences 



