258 THE USE OF THE MICROSCOPE 



without a cover, the sharp ring is seen within the focus of 

 the microscope, and beyond the focus is a vague mist. 

 This shows undercorrection of the objective, and the tube 

 should be lengthened to the right degree. With the cover- 

 glass of 0.23 millimeter thickness, the sharp ring is beyond 

 the focus of the microscope, and the misty spot within the 

 focus. This overcorrection can be adjusted by shortening 

 the tube sufficiently. (The Abbe test plate often shows 

 some minute holes in the silver film, which may serve for the 

 star test.) (Siedentopf, Coles.) 



25. Adjusting Tube Length with Dry Objectives. — Take 

 a monocular microscope with a drawtube and a corrected 

 and well-adjusted condenser. Measure the tube length, 

 from the rim of the drawtube (or fixed top tube) to the sur- 

 face against which the shoulder of the objective fits. See 

 if the graduation on the drawtube includes the revolving 

 nosepiece, or does not include it, or includes a nosepiece of 

 different thickness. Make the tube length 160 millimeters 

 (for most objectives), or 170 millimeters (for the objectives 

 of Leitz). Take a 20 apochromatic objective, giving best 

 images with a cover-glass 0.17 millimeter thick. Use it 

 with a % or %o cone, and a 20 eyepiece. Put the standard 

 cover-glass on a dry well-stained bacterial smear, and 

 observe definition and resolution. Remove the cover-glass, 

 and note that the drawtube requires to be lengthened by 

 several centimeters (about 8)2 centimeters) to get good 

 definition again. 



Take a 40 objective of 0.85 aperture corrected for a 

 cover 0.17 millimeter thick. Test it on the above object 

 with standard cover, with all adjustments correct and a 

 %o condenser cone. Change the cover-glass for one 0.15 

 millimeter thick. Note that the drawtube has to be pulled 

 out about 20 millimeters to get good definition. (Nelson, 

 Spitta, Coles.) 



26. Measuring Cover-glasses for the Water-immersion 

 Objective. — Take a No. 1 cover-glass below 0.17 millimeter 

 thick. Run the screw gage (with a ''feeler" milled head) 

 down to see if the zero is correct. Allow for any slight 



