PRIESTLEY SMITH S PERIMETER. 



771 



which a needle projects, and which is the zero of the instrument. A carriage (i), in which the 

 test objects are placed, can be moved in the concave face of the quadrant by means of the milled 

 head (/), which moves the carriage by means of a tooth and pinion wheel.] 



[When the milled head (j) is turned, it moves the carriage and two slides (k and I), the two 

 slides moving in the ratio of 2 to 1. The rate of the carriage is so adjusted that it travels ten 

 times faster than I, and five times faster than k. The pointer (p) is connected with these slides, 

 so that it moves when they move, and records its movements by piercing the record chart, 

 which is fixed in the double-faced frame (e). The frame for the record chart is hinged near c 

 to the upright (D). The frame, when upright, comes so near the pointer that the latter can 

 pierce a chart placed in the frame. The patient is directed to look at the "fixation point," 

 which is merely a small ivory button placed in the imaginary axis of the hemisphere on the 



Fig. 558. 

 M 'Hardy's perimeter. I, porcelain button ; M, bit; E, for fixing the head ; g, h, quadrant ; 

 0, fixation point ; p, pointer for piercing the record chart held in the frame (e) which moves 

 on c ; D, upright supporting the quadrant and the automatic arrangement of slides (k and 

 I), which are moved by/. 



front of the centre of the concave surface of the quadrant ; the projecting needle-point (0) indi- 

 cates its position. This is the zero of the quadrant, and on each side of it the quadrant is 

 divided into 90.] 



[In testing the field of vision, place the carriage so as to cover zero, adjust the eye for the 

 fixation point, and look steadily at it, when, if all is right, the pointer (p) ought to pierce the 

 centre of the chart. Move the carriage along the quadrant by/ until it disappears from the 

 field of vision, and when it does so the pointer is made to pierce the chart. Make another 

 observation in another direction by altering the position of the quadrant, and go on doing so 

 until a complete record is obtained of the field of vision. Test the other eye in the same way. 

 The colour-field may be tested by using coloured papers in the carriage.] 



[Priestley Smith's perimeter (fig. 559). The wooden knob on the left of the figure is placed 

 under the eye of the patient, who stares at the fixed point in the axis of the quadrant, which 

 can be moved in any meridian. The test object is a square piece of white paper, which is moved 

 along the quadrant. The chart is placed on the posterior surface of the hand-wheel and moves 

 with it, so that the meridians of the chart move with the quadrant. There is a scale behind 

 the hand-wheel corresponding with the circles on the chart, so that the observer can prick off 

 his observations directly.] 



