338 



PHYSIOLOGY 



CHAP. 



By its means a small white or coloured object can be presented to 

 the subject in any meridian of the field and at any distance from 

 the fixation point. 



Forster's perimeter (Fig. 162), on which all others have been modelled, 

 consists of a half-circle B of metal, marked with a scale. A runner to which 

 small white or coloured discs can be fastened moves along its blackened 

 concave surface. The middle of the arc, which is fixed in a horizontal axis 

 around which it can rotate, forms the point of fixation. An indicator G 

 shows the position of the meridian examined on the opposite side of the axis. 



80 



70 



60 



..48--. 



too 



10 . 



gfo rto' 





X. - 



/*- 



i ""* 



10 



-*' 



50 . ak> 7!0 



i i 





eo 



FIG. 163. Normal visual field of ri^lit eye of an individual aged 40, with a white square of 

 20 sij. mm. (Luciani.) 



The subject rests his chin on a support H. The perimeter is graded so t 

 the zero point is in the centre. 



The subject is so placed that the eye to be examined is at the level of the 

 centre of the perimeter, and he is asked to look at the fixation point, the other 

 eye being blindfolded to avoid confusion. The runner with the disc is then 

 moved slowly from the periphery to the centre, and the subject is asked to 

 indicate the moment at which he first sees it, while his eye remains fixed on 

 the centre of the arc. The corresponding degree is then read on the convex 

 surface of the arc, and the position of the meridian is also noted. When this 

 operation is repeated in different meridians the field of vision can be repre- 

 sented graphically on a chart (Fig. 163). The zero corresponding to the 



hat 

 ^u. 



