23 MICROSCOPE AND ACCESSORIES. 





In learning to adjust objectives, it is best for the student to choose 

 some object whose structure is well agreed upon, and then to practice 

 lighting it, shading the stage and adjusting the objective, until the 

 proper appearance is obtained. The adjustment is made by turning a 

 ring or collar which acts on a screw and increases or diminishes the dis- 

 tance between the systems of lenses, usually the front and the back 

 systems (Fig. 13). In adjustable objectives the back systems should be 

 movable, the front one remaining fixed so that there will be no danger 

 of bringing the objective down upon the object. If the front system is 

 movable, the body of the microscope should be raised slightly every 

 time the adjustment is altered. 



Ge?ieral Directions. — (A) The thinner the cover-glass the further 

 must the systems of lenses be separated, i. e., the adjusting collar is 

 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. III). 



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

 ment are given by Mr. Wenham [(C. 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 posi- 

 tion. 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 -without the focus, or farthest from the ob- 

 jective [i. <?., in focusing up], the lenses must be placed further asunder, 

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

 the zero mark as the cover-glass is too thin for the present adjustment]. 

 If the greater expansion is when the object is within the focus, or near- 

 est the objective, [z. <?., 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 grad- 

 uated arbitrarily, the zero (O) mark representing the position for uncov- 

 ered objects. Other objectives have the collar graduated to correspo7id to 

 the various thickness of cover-glasses for which the objective may be ad- 

 justed. This seems to be an admirable plan ; then if one knows the 

 thickness of the cover-glass on the preparation {Ch. V) the adjusting col- 

 lar 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 mount- 

 ing 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 



