ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
227 
Tlie polarizer is fitted to an arm in the substage which swings in and 
out, and has a revolving fitting with spring catches to hold it at the 
points when the prisms are crossed ; an achromatic system of lenses is 
supplied to fit in over the polarizer, and is easily adjusted in the sleeve 
for axial illumination. 
The analyser is fitted over the eye-piece, which is supplied with 
cross wires and is in a rotating fitting similar to that of the polarizer ; 
^being provided with spring detents, it revolves freely over the eye-piece, 
and has a graduated scale of degrees on its circumference. 
Messrs. It. & J. Beck’s Large Model Petrological Microscope. — 
This instrument has been specially designed for advanced petrological 
work (fig. 31). 
It has a rack-and-pinion coarse-adjustment and micrometer screw fine- 
adjustment. The draw-tube carries a slide containing a Bertrand lens, 
which can be pushed in and out, and is provided with a rack-and-pinion 
adjustment for focusing it, when examining the rings and brushes of 
•crystals. 
The centering arrangement is supplied to the nose-piece, or may be 
added to the stage instead if desired. 
Q 2 
