THE COVER-CLASS I'ROBLEM 111 



aperture and, hence, of useful magnification. On tlie 

 other hand, differences of cover-glass thickness (and covers 

 in boxes marked No. 1 may sometimes vary from 0.09 to 

 0.21 millimeter in thickness), if near to or more than 0.03 

 miUimeter from the standard, blur the image perceptibly, 

 even in medium-power work. (The standard cover-glass 

 thickness, as already stated, is most often 0.17 millimeter.) 

 The loss to the image may be seen (1) by substituting 

 a cover-glass of standard thickness, or (2) by employing a 

 medium oil-immersion objective, of about the same focal 

 length and aperture as the dry objective, on the object 

 in question. For the oil-immersion objective is not so 

 much affected by differences in cover-glass thickness as 

 are dry or water-immersion objectives. 



Remedies. — The blurring with the 4-minimeter objective 

 can be remedied in several ways: 



1. Cover-glasses near, or precisely, the standard thick- 

 ness can be purchased for a small multiple of the cost of 

 ordinary covers. This is probably the best remedy. 



2. Number 1 (or No. 2) covers may be measured with 

 a screw gage, and those at or shghtly under the standard 

 thickness selected for use on the objects most studied. 

 Those thinner than this can be used for routine work with 

 balsam mounts and oil-immersion objectives; and those 

 thicker can be employed only for low-power work, or may 

 be rejected. 



3. If cover-glasses are selected between about 0.13 and 

 0.18 millimeter thick, a fair correction can usually be made 

 by the sliding tube of the microscope. (But this method 

 is rather antiquated, since covers close to the standard 

 can now be obtained.) There should be, usually, 160 

 millimeters of tube length (from the shoulder of the objective 

 to the upper edge of the drawtube) for covers of standard 

 thickness. To correct the 40 objective of 0.85 aperture, 

 for every 0.01 millimeter of cover-glass thickness (plus 

 mounting medium over object) less than 0.17 millimeter, the 

 tube is to be pulled out about 10 millimeters, and vice versa. 

 This amount may be determined accurately by the star 



