226 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
tlie exact length of 160 mm. for which all their objectives have been 
adjusted. When a revolver is used, which is a useful adjunct to this 
stand, the tube should be shortened 15 mm., which is the depth of the 
collar of the revolver. The correct position of the tube when a revolver 
is used may be seen by a mark cut into the draw-tube. 
Messrs. B. & J. Beck’s New Student’s Petrological Microscope- 
Figs. 29 and 30 illustrate two forms of a cheap Petrological Microscope 
recently made by Messrs. Beck. 
The only difference in the construction of these two instruments is 
that one has a rack-and-pinion, and the other a sliding coarse-adjustment. 
The principal feature in the construction of a small model Microscope 
Fig. 29 . 
is the fine-adjustment, which in these instruments, the makers affirm, is 
exceedingly simple, yet perfectly efficient. 
In the present instrument the stage is supplied with a centering 
adjustment, revolves concentrically with the optic axis, and has a gradu- 
ated scale of degrees on the circumference, with finders in horizontal 
and vertical directions. 
