THE PERIMETER 



225 



A person who has been proved colour blind by the first test, but 

 who only selects purples in the second, is incompletely colour blind. 



If he select with the purples blue and violet, he is completely red 

 blind. 



If he select with purple only green and grey, he is completely 

 green blind. 



As a final and confirmatory test, the third test skein is presented. 



III. This is a bright red skein of medium shade tending rather 

 to a yellowish red. 



The red-blind person chooses with the reds, greens and browns 

 of darker shade than the skein presented. 



The green blind chooses with the reds, greens and browns of 

 lighter shade than that of the test skein. 



PERIMETRY 



When we wish to carefully examine any object we turn our eyes 

 to such a position that the image of the object falls upon the fovea 

 centralis. This is termed direct vision. The vision produced by 

 images formed on the peripheral parts of the retina is in contradis- 

 tinction termed indirect vision. Indirect vision is much less acute 

 than direct, but still the periphery of the retina is capable of appre- 

 ciating movements or changes in intensity of light falling upon it. 



In order to test the limits 

 of indirect vision the peri- 

 meter (fig. 176) is employed. 

 This consists of a vertical 

 pillar carrying a horizontal 

 axis which bears on one side 

 a circular arc and on the 

 other a vertical disc to which 

 a chart can be fixed. A little 

 holder can be moved along 

 the arc into any position. A 

 second support terminating in 

 a knob is provided, against 

 which the cheek is held about 

 an inch below the centre of 

 the eye to be tested. The 

 opposite eye is then closed, 

 and the other one fixed in 

 position by looking steadily 

 at a white knob in the centre 

 of the axis of the instrument. 

 A chart is next fixed in posi 



Fig. 170. 



-Priestley-Smith's Perimeter. 

 (Halliburton.) 



tion in the vertical disc, and first a white square held in the carrier on the 

 rotating arc. This is moved from the periphery towards the centre until it 

 is clearly observed, and its position then marked upon the chart. The arc 



Q 



