66 LIGHTING AND FOCUSING [CH. II 



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 posi- 

 tion for uncovered objects. Other objectives have the collar graduated to 

 correspond to the various thickness of cover-glasses for which the ob- 

 jective may be adjusted. This seems to be an admirable plan; then if 

 one knows the thickeness of the cover-glass on the preparation (Ch. 

 VIII ) the adjusting collar may be set at a corresponding mark, and 

 one will feel confident that the adjustment will be approximately cor- 

 rect. It is then only necessary for the observer to make the slight ad- 

 justment 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 remember that : (A) If the tube of the microscope is longer 

 than the standard for which the objective was corrected, the effect 

 is approximately the same as thickening the cover-glass, and there- 

 fore the systems of the objective must be brought closer together, i. 

 e., the adjusting collar must be turned away from the zero mark. 

 (B) 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. 45). 



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 standard. The plan of designating exactly what is meant 

 by "tube-length," and engraving on each objective the "tube- 

 length for which it is corrected, is to be commended, for it is mani- 

 festly difficult for each worker with the microscope to find out for 

 himself for what "tube-length" each of his objectives was cor- 

 rected. (See Ch. X.) 



§ 115. Water Immersion Objectives. — Put a water immer- 

 sion objective in position (§ 54) and the fly's wing for object under 



