344 



PRACTICAL PHYSIOLOGY. 



[lxxiil 



LESSON LXXIIL 



PERIMETRY— IRRADIATION— IMPERFECT VISUAL 

 JUDGMENTS. 



1. To Map out the Field of Vision, or Perimetry. 



('/.) A rough method is to place the person with his back to a 

 window, ask him to close one eye, stand in front of him about 2 

 feet distant, hold up the forefingers of both hands in front of and 



in the plane of your own 

 face. Ask the person to 

 look steadily at your nose, 

 and as he does so observe 

 to what extent the fingers 

 can be separated horizon- 

 tally, vertically, and in 

 oblique directions before 

 they disappear from his 

 field of vision. 



{b.) Priestley Smith's Peri- 

 meter (fig. 262).— Let the ob- 

 server seat himself near a 

 table on which the perimeter 

 is placed at a convenient 

 height. Suppose the right eye 

 is to be examined, fix a blank 

 chart for the right eye behind 

 the wooden circular disc. A 

 mark on the hand-wheel shows 

 which way the chart is to be 

 l)laced. 



(('.) The patient rests his 

 riglit cheek against the knob 

 on the wooden pillar in siich a 

 position that the knob is about 

 an inch directlj under his right eye, the other eye is closed either voluntarily 

 or with a shade, while the observer looks steadily with the right eye at the 

 white spot on the end of the axis of the instrument. 



(d.) The observer turns the quadrant with his right hand by means of the 

 wooden wheel, first to one and then to another meridian. With his left he 

 moves the white mark along the quadrant, beginning at the periphery and 

 gradually approaching centralwards until it is just seen by the right eye. A 

 prick is then made in the chart corresponding to the angle read off on the 

 quadrant, at which the observer can see the white spot. 



(e.) Turn the quadrant to another meridian and determine the limit of the 

 risual field as before. This is repeated for four or more meridians, and theo 



262.— Priestley Smith's Perimeter. 



