142 THE USE OF THE MICROSCOPE 



marked on the tube should be compared with a scale; for 

 some optical firms do, and some do not, allow in the num- 

 bering of the divisions for the presence of a nosepiece. 

 The first thing to do on setting up a microscope with a 

 drawtube is to put the tube length at 160 or 170 miUimeters 

 (according to the maker). This task is avoided if the 

 drawtube is omitted. 



Use of the Drawtube. — In the preparation of slides 

 (in iron-acetocarmine) the writer has used an 11 -millimeter 

 objective of 0.35 aperture. It gave poor images with the 

 uncovered preparations, but when the tube was pulled 

 out for about 2.5 centimeters more, the picture was sharp. 

 The excellent apochromatic 20 (8 millimeters) gives poorer 

 definition with covers which deviate 0.02 from the standard 

 thickness of 0.17 millimeter; but can readily be adjusted 

 by extending the tube length about 2 miUimeters for each 

 0.01 in deficit of the cover. But the dry 40 (4.3 millimeters) 

 of 0.85 aperture, a fluorite objective, as already stated 

 is most sensitive to differences in the thickness of the cover- 

 glass, or mounting medium. Measuring the cover, and 

 increasing the tube length about 10 millimeters for every 

 0.01 milUmeter below the standard cover-glass, gives good 

 results. When a ^{2 oil immersion is employed on objects 

 in water, the pulling out of the tube for an appropriate 

 distance (usually up to 10 millimeters) improves the vision 

 of objects which are slightly below the cover-glass. The 

 3-millimeter oil-immersion is corrected by shorter increase 

 of tube length than the 2-millimeter oil. (The 2-millimeter 

 dry, on the other hand, according to the maker, is not 

 perceptibly affected by the ordinary changes of tube length, 

 and cannot thus be corrected for different cover 

 thicknesses.) 



In the correction collar, the back lenses are moved 

 closer to the front lenses for thicker covers, and moved 

 farther away for thinner covers; just as the eyepiece is 

 moved closer to the objective for thicker, and farther away 

 for thinner covers, when the drawtube is used for the 

 correction. Thus for thicker covers the front lens of the 



