56 LIGHTING AND FOCUSING \CH. II 



the cover-glass being too thick for the present adjustment]." In most 

 objectives the collar is graduated arbitrarily , the zero (O) ?nark represent- 

 ing the position for tincovered objects. Other objectives have the collar 

 graduated to correspotid to the various thicknesses of cover-glasses for which 

 the objective may be adjusted. This seems to be an admirable plan ; then 

 if one knows the thickfiess of the cover-glass on the preparation (Ch. VIII) 

 the adjusting collar may be set at a corresponding mark, and o?ie will feel 

 confident that the adjustment will be approximately correct. It is then 

 o?ily 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 : (A) If the 

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

 jective was corrected, the effect is approximately the same as thicken- 

 ing the cover-glass, and therefore the system 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. 40). 



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 mil- 

 limeters, 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 this 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 manifestly difficult for each worker with 

 the microscope to find out for himself for what "tube-length" each of 

 his objectives was corrected. (See Ch. X). 



§ 104. Water Immersion Objectives. — Put a water immersion 

 objective in position (§ 47) and the fly's wing for object under the 

 microscope. Place a drop of distilled water on the cover-glass, and 

 with the coarse adjustment lower the tube till the objective dips into 

 the water, then light the field well and turn the fine adjustment one 

 way and another till the image is clear. Water immersions are exceed- 

 ingly convenient in studying the circulation of the blood, and for 



