CH. Ill] ADJUSTMENT OF OBJECTIVES 79 



object is within the focus, or nearest the objective [i.e., in focusing 

 down], the lenses must be brought closer together, or toward the mark 

 covered [i.e., the adjusting collar should be turned away from the 

 zero mark, the cover-glass being too thick for the present adjustment]." 

 In most objectives the collar is graduated arbitrarily, the zero (o) 

 mark representing the position for uncovered objects. Other objec- 

 tives have the collar graduated to correspond to the various thickness 

 of cover-glasses for which the objective may be adjusted. This seems 

 to be an admirable plan ; then if one knows the thickness of the cover- 

 glass on the preparation (Chs. IX-X) the adjusting collar may be set 

 at a corresponding mark, and one will feel confident that the adjust- 

 ment will be approximately correct. It is then only necessary for 

 the observer to make the slight adjustment to compensate for the 

 mounting medium or any variation from the standard length of the 

 tube of the microscope. In adjusting for variations of the length of 

 the tube from the standard it should be remembered that: (i) If 

 the tube of the microscope is longer than the standard for which the 

 objective was corrected, the effect is approximately the same as thick- 

 ening the cover-glass, and therefore the systems of the objective must 

 be brought closer together, i.e., the adjusting collar must be turned 

 away from the zero mark. (2) If the tube is shorter than the standard 

 for which the objective is corrected, the effect is approximately the 

 same as diminishing the thickness of the cover-glass, and the systems 

 must therefore be separated (fig. 35), i.e., turned toward the zero 

 mark. 



In using the tube- length for cover correction shorten the tube for 

 too thick covers, and lengthen the tube for too thin covers. 



Furthermore, whatever the interpretation by different opticians 

 of what should be included in tube-length, and the exact length in 

 millimeters, its importance is very great, for each objective gives the 

 most perfect image of which it is capable with the tube-length for 

 which it is corrected, and the more perfect the objective the greater 

 the ill-effects on the image of varying the tube-length from the stand- 

 ard. The plan of designating exactly what is meant by tube-length 

 and engraving on each objective the tube-length for which it is cor- 

 rected, is to be commended, for it is manifestly difficult for each worker 



