'74 THE EYE AND VISION. [CH. LV. 



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



Fig. 589 shows one of the forms of perimeter very generally 

 employed, and fig. 590 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 

 for different colours ; thus a blue object is seen over a wider 

 field than a red, and a red over a wider field than a green object. 



In disease of the optic nerve, contraction of the field of vision for 

 white and coloured objects is found. This is often seen before any 

 change in the optic nerve is discoverable by the ophthalmoscope. 



The Fovea Centralis. 



This is the region of most acute vision ; when we want to see 

 an object distinctly we look straight a*t it. It is also the region 

 where the colours of objects are best distinguishable. It is, 

 however, stated to be less sensitive from one point of view than 

 the zone immediately surrounding it ; that is to say, the minimum 

 intensity of white light which will cause an impression is some- 

 what greater. But with this exception, the sensibility of the 

 retina diminishes steadily from centre to circumference. 



The yellow spot of one's own eye can be rendered evident by 

 what is called Clerk-Maxwell's experiment : on looking through 

 a solution of chrome alum in a bottle with parallel sides, an oval 

 purplish spot is seen in the green colour of the alum. This is 

 due to the pigment of the yellow spot. 



