ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
879 
of the slide through the length of the scale. Secondly, the double wire 
of the micrometer must be directed parallel to the marks of the division 
by turning the body-tube, after loosening the screws of the Microscope- 
holder. Thirdly, for avoidance of parallax, the image of the division 
must be brought exactly in the plane of the double wire. 
Fig. 101. 
For the adjustment of Microscope II. the slide is moved so far to 
the left that the first divisions of the scale come beneath the Microscope. 
The double wire is then directed parallel to the marks of the division, 
