344 OPTICAL PROJECTION 



between H and K. The whole instrument is about 24 inches 

 long. 



For convenience at different distances, it is well to have 

 a polariscope fitted with several powers. I have a second 

 back lens of 6-inch focus, as well as the usual 5-inch back 

 lens ; then, by removing the front lens of the power when 

 needed, there will be a range of powers of about 3|, 4, 5, and 

 6 inches. They are easily adjusted by screwing in a lengthen- 

 ing ring or adapter. Another plan often adopted is to have a 

 concave amplifying lens mounted in front of the instrument, 

 which amplifies the image precisely as described in 93, 

 Chapter XIII. 



An alum-cell may be used in the ordinary lantern-stage 

 with this polariscope, but is not really necessary. The glas? 

 caps protecting the Nicol should be removed for a few minutes 

 after lighting up, in order to allow the dew to escape which 

 will at first form on the end surfaces. In ordinary work the 

 stage and power s F is pushed close up to the end of the 

 polariser, and not kept apart from it, as here shown for 

 clearness. Occasionally a tube of fluid may be inserted 

 between the polariser and the stage, then the stage with power 

 is drawn forward sufficiently to allow of this. 



It is very easy to adapt a low microscopic power to this 

 polariscope, by fitting a tube into the nozzle of F with a slot 

 or slide-stage at the point where the lenses F condense the 

 light into the smallest area. In the other end of this tube 

 slides (for focussing) a power of about two inches, and the 

 analyser. Such a power will exhibit many fine rock sections 

 or ordinary micro slides, which the ordinary 3^-inch power 

 is unable to do. But for anything like real microscopic work, 

 resort must be had to the polarising microscope described in 

 Chapter XIII. 



Great care should be taken of fine calcite prisms. The 

 ends should be cleansed only with a clean, round-nosed camel- 

 hair brush, often called a * sky-brush ' at artists' shops. If 



