CH. IT' 



LIGHTING AND FOCUSING 



55 



which the objective is capable by selecting covers of the thickness for which the 

 objective was corrected. (See table p. 14.) Adjustment maybe made also by 

 increasing the tube-length for covers thinner than the standard and by shortening 

 the tube-length for covets thicker than the standard (Fig. 58). 



Fig. 57. Effect of the cover-glass 

 on the rays from the object to the 

 objective (Ross). 



Axis. The projection of the optic 

 axis of the microscope. 



F. Focal or axial point of the 

 objective. 



F' and F" . Points on the axis 

 where rays 2 and 3 appear to originate 

 if traced backward after emerging 

 from the upper side of the cover-glass. 



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, shad- 

 ing 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 distance between the systems of lenses, usually the 

 front and the back systems (Fig. 40). 



§ 103. General Directions. — (A) The thinner the cover-glass, the 

 further must the system of lenses be separated, i. <?., 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. IV). 



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

 ment are given by Mr. Wenham (Carpenter, 7th Ed., p. 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 without the focus, or 

 farthest from the objective [/. e., in focusing up], the lenses must be 

 placed further asunder, or toward the mark uncovered [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 nearest the objective [z, e., in focusing down], 

 the lenses must be brought closer together, or toward the mark covered 

 [z. e., the adjusting collar should be turned away from the zero mark, 



