770 



m 4 hardy's perimeter. 



former we can circumscribe the surface of a hemisphere, in the centre of which the eye is placed 

 Proceeding from the fixed point, objects are placed upon semicircles, and are gradually pushed 

 more and more towards the periphery of the field of vision until the object becomes indistinct, 

 and finally disappears. The process of testing is continued by placing the arc successively m 

 the different meridians of the field of vision. .*'** v + in 



[VL 'Hardy's perimeter is a very convenient form (fig. 568). It consists of two uprights (L 

 and D) which are fixed to the opposite ends of a flat basal plate (A). C carries an arrangement 

 for supporting the patient's head, while D carries the automatic arrangement for the perimetric 



Fig. 557. 

 Horizontal section of the right eye. a, cornea ; b, conjunctiva ; c, sclerotic ; d, anterior 

 chamber containing the aqueous humour; e, iris;//', pupil; g, posterior chamber; I, 

 Petit's canal ; /, ciliary muscle ; k, corneo-scleral limit ; i, canal of Schlemm ; m, choroid ; 

 , retina; o, vitreous humour ; No, optic nerve; q, nerve-sheaths; p, nerve-fibres; Ic, 

 lamina cribrosa. The line A indicates the optic axis ; S r the axis of vision ; r, the 

 position of the fovea centralis. 



record. Both of these can be raised or depressed by the screws (G and b). The patient's chin 

 rests on the chin-rest (E), while in the mouth is placed Landolt's biting fixation (L), which is 

 detachable. The position of the head can be altered by sliding F on L, which can be fixed in 

 any position by the screw (0). The porcelain button (I) just below the patient's eye (I) is con- 

 nected with the adjustment of the "fixation point." The automatic recording apparatus 

 consists of a revolving quadrant (h, h), which describes a hemisphere round a horizontal axis 

 passing through the centre of the hollow male axle, turning in the female end of a, which is 

 supported by 1). The quadrant can be fixed at any point by g. On the front concave surface 

 of the quadrant is fixed a circular white piece of ivory, representing the ' fixation point," from 



