40 Modern Microscopy 



be mentioned very excellent models by Swift and Son, 

 E. and J. Beck, Leitz, and Watson and Sons, the last-named 

 offering to construct a body-tube of a size to carry eye- 

 pieces of any desired gauge, so as to save duplication of 

 apparatus. ' The instrument illustrated in Fig. 13, by 

 Swift, is of ingenious and exceedingly compact and efficient 

 design. 



PETROLOGICAL MICROSCOPES. 



A variety of microscopes are made for this purpose only, 

 but those who may wish to examine rocks and crystals and 

 to use their instrument for ordinary work as well will find 

 it advisable to have a concentric rotating stage with the 

 periphery divided and to read by verniers, a polarizer 

 having a divided rotating circle, and immediately above it 

 and fitting in the sub-stage with it a condenser of large 

 aperture. The analyzer may be arranged to fit together 

 with divided circles over the eyepiece of the instrument. 

 Cross-webs can be fixed to the diaphragm of the eyepiece, 

 and a calcspar plate can be fitted immediately beneath the 

 analyzer prism in its carrier over the eyepiece. In the 

 regular petrological microscopes these arrangements are 

 already made, and in such, an extra analyzer is usually 

 mounted in a box in the body of the microscope in such a 

 manner that it can be pushed out of the field of view when 

 desired. For petrological study exclusively, the instrument 

 designed by Mr. Allan B. Dick and manufactured by Swift 

 and Son, is usually conceded to be the most efficient pattern 

 that is made. The special feature of this is that the stage is 

 fixed, and instead of rotating the object, the polarizing and 

 analyzer prisms with the eyepiece, are made to revolve 

 together. Much of the time which ordinarily is occupied 

 in effecting the exact centring of the stage to insure con- 

 centricity of revolution is thereby obviated. 



