IRRADIATION. 279 



(6.) Priestley Smith's Perimeter (Fig. 129). Let the 

 observer 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 ihe chart is to be placed. 



(c.) The patient rests his right cheek against the knob on 

 the wooden pillar in such a position that the knob is about 

 an inch directly 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 deter- 

 mine the limit of the visual field as before. This is 

 repeated for four or more meridians, and then the pricks on 

 the chart are joined by a continuous line, when we obtain an 

 oval field more extensive in the outer and lower portions. 

 Test, if desired, the left eye, substituting a blank chart for 

 that eye. 



(f.) Test the field of vision for colours, substituting for 

 the white travelling disc, blue, red, and green. Mark each 

 colour-field on the chart with a pencil of similar colour. 

 Notice that the field for blue is nearly as large as the 

 normal visual field. It is smallest for green, red being 

 intermediate between green and blue. 



(g.) With Lud wig's apparatus test when red, yellow, blue, 

 and other coloured glasses cease to be distinguished as such 

 in the field of vision. 



2. Irradiation. By irradiation is meant the fact that, under 

 certain circumstances, objects appear larger than they should be 



