MIRROR, STAGE, SOSEPIECE, AM) DRAWTUBE 145 



cases, but this does not always need to be fitted with 

 verniers and scales. 



The revolving nosepiece should be kept accurately 

 centered and revolved only in the right direction. The 

 high objectives should each be centered fairly accurately, 

 and the low objectives should be made concentric with the 

 high ones. Lack of centering of the high objectives is 

 one of the worst errors of microscopy, and spoils all images, 

 more or less. 



The drawtube is doubtless unnecessary on instruments 

 for routine work, and it may be a source of errors of adjust- 

 ment. Its use may be avoided by employing cover-glasses 

 of standard thickness. A 14-millimeter diaphragm, 1 

 centimeter below the lowest eyepiece is always useful 

 with a large source of Hght (Nelson). A drawtube is 

 handy on both monocular and binocular for the most 

 accurate adjustments, in the absence of correction collars. 

 Such corrections may often be made, however, by pulling 

 out the eyepieces more or less. No correction by the draw- 

 tube is quite equal to the use of the correct cover thickness. 



Practical Points 



1. Use a corrected condenser for apertures above 0.2, instead 

 of a concave mirror. 



2. For high-power work, the glass plane mirror may well be 

 replaced by a silvered reflecting prism, which gives only one 

 image of the source. 



3. The reflecting surface of prism or mirror should be centered 

 with the optic axis, and the source of light should be central, too. 

 This is especially needful with a 25-millimeter prism, and a high 

 objective. 



4. A sliding bar on a square stage enables slides to be system- 

 atically searched, or the Detto sliding bar may be used on a round 

 stage. 



5. An object traverser (mechanical stage) is useful for search- 

 ing, and as a finder; but the scales and verniers may often bo 

 dispensed with. 



6. A revolving nosepiece for several objectives is essential 

 for rapid work, and nmst be kept accurately centered. 



