CH. II.] LIGHTING AND FOCUSING. 53 



turned nearer the zero or the mark " uncovered," and conversely ; (B) 

 the thicker the cover-glass the closer together are the systems brought 

 by turning the adjusting collar from the zero mark. This also increases 

 the magnification of the objective (Ch. IV). 



The following specific directions for making the cover-glass adjust- 

 ment are given by Mr. Wenham (Carpenter, 166). " Select any dark 

 speck or opaque portion of the object, and bring the outline into perfect 

 focus ; then lay the finger on the milled-head of the fine motion, and 

 move it briskly backwards and forwards in both directions from the first 

 position. Observe the expansion of the dark outline of the object, both 

 when within and when without the focus. If the greater expansion or 

 coma is when the object is ivitkout the focus, or farthest from the ob- 

 jective [/. e. , in focusing up] , the lenses must be placed further asunder, 

 or toward the mark uncovered [z. e. , the adjusting collar is turned 

 toward the zero mark as the cover-glass is too thin for the present ad- 

 justment]. If the greater expansion is when the object is within the 

 focus, or nearest the objective, [z. e., in focusing down] , the lenses must 

 be brought closer together, or toward the mark covered, [z. e. , the ad- 

 justing 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 (<9) mark representi?ig the 

 position for uncovered objects. Other objectives have the collar graduated 

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

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

 knows the thickness 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 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 : (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 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. 40). 



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

 too thick covers and Lengthen the tube for too thin covers. 



