51 
adjustable stage (fig. 2c and fig. 4), and, finally, (4) a support (fig. 2d). 
The cross wire in the eyepiece furnishes a point of reference, and in 
making a reading the eye is first oriented so as to bring its optical 
Fig. 3. — Micrometer slide. Micrometer screw has a diameter of 10 mm., pitch of 0.5 mm. The microm- 
eter head has a diameter of 50 mm. and is divided into 100 parts, each division reading to 0.005 mm. 
axis parallel with the axis of the microscope. Then by means of a 
coarse adjustment the desired edge of the pupil is brought into line 
with the cross wire and the long axis of the pupil measured; the 
object stage is rotated to 90° and the short axis measured. In this 
way a given eye can be measured many times without disturbing it 
