32 PHOTOMICROGRAPHY 



over is obtained by screwing the lens into the bottom 

 of the draw-tube, which can be racked further away 

 from the stage than the outer tube. 



The Fine Adjustment. The fine adjustment calls for 

 most careful attention when choosing a microscope, 

 and can only be dispensed with for very low powers. 

 Most makers have a form of their own, but it is un- 

 necessary to describe them in detail, as the catalogues 

 generally give figures and explanation of the types 

 employed, and a collected description of the principal 

 patterns will be found in Spitta's Microscopy. Most of 

 them depend primarily on a micrometer screw which 

 causes the tube to move up or down on a slide. The 

 slide in the typical Continental pattern (Fig. 9) takes the 

 form of a sleeve fitting over a triangular prismatic bar, 

 and the micrometer screw generally bears directly on 

 the prism, and moves both the tube and limb. This is 

 a very smoothly acting arrangement, but, in some forms, 

 is open to the objection that, if the microscope is lifted by 

 the limb, the micrometer screw may be damaged. Several 

 manufacturers make special provision against this how- 

 ever, by completely casing the prismatic bar and slide, or 

 by attaching a small handle to the solid part at its base. 

 But, in any case, the pitch of a direct-acting screw is 

 limited to about T V or T J^ of an inch, in order that it 

 may withstand wear and tear, and carry the weight of 

 the limb and body. The motion such a screw imparts 

 is too rapid for high-power wide-angle objectives. One 

 rotation of the milled head usually raises or lowers the 

 tube 5*0 or T V of an inch, the screw of -^ inch pitch being 

 less frequently used. 



In other microscopes the slide is a dovetail fitting 

 carrying the tube and coarse adjustment only, and 

 the limb may be lengthened to make room for a larger 

 stage, or strengthened to form a handle, without adding 

 to the weight that the fine adjustment has to lift. Motion 



