The Body-Tube 29 



upon it to parts of a centimetre or inch. The object of 

 these divisions is to enable a record to be kept of magni- 

 fications at different points of extension, or a note to be 

 made of the lengths of tube that give the most perfect 

 corrections for certain objects and objectives. 



In all microscopes of medium or high-class, the universal 

 thread should be fitted to the lower end of the draw-tube ; 

 where there are two draw-tubes it should be supplied to the 

 outer one. The advantages of this adapter are numerous. 

 A low-power objective can be used in it which it is often 

 impossible to focus on many stands, owing to the compact- 

 ness of the build and shortness of the movement of the 

 coarse adjustment. With the two draw-tubes, if the outer 

 one have this adapter fitted, nearly 10 inches of separation 

 can be obtained between the eyepiece and the objective. 

 It is further useful for carrying the apertometer objective 

 and the analyzer, described respectively on pages 63 and 

 102 ; also the Bertrand's lens for examining the ' brushes ' 

 of crystals, and for many other purposes. 



It has occurred within the experience of the writer that 

 results obtained on a microscope having a large tube could 

 not be reproduced with the same objective on an instru- 

 ment having a small tube. This was traced to be due to 

 the diaphragm at the bottom of the draw-tube, and it has 

 since been found that in many students' stands the opening 

 of this diaphragm is as small as f inch. This is altogether 

 insufficient, and causes restriction to the passage of rays 

 from the objective. It would be well to see that this 

 diaphragm had an opening of at least f inch. 



TAILPIECE AND MIRRORS. 



The Tailpiece. — It will usually be found convenient if 

 the arm, or tailpiece, which carries the mirrors, be so 

 mounted as to be turned aside with the mirrors when 

 desired. . This arrangement is of great utility, for it permits 

 of light being readily directed from the lamp through the 



