CH. 11} 



LIGHTING AND FOCUS l.\(, 



standard and by shortening the tube-length for covers thicker than the 

 standard (Fig. 65). 



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 distance between the systems of lenses, 

 usually the front and the back systems (Fig. 45). 



FIG. 64. Effect of the cover- 

 g/ass 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 a.ris 

 where rays 2 and j appear to orig- 

 inate if traced backward after 

 emerging from the upper side of 

 the cover-glass. 



114. Directions for Adjustment. (A) The thinner the 

 cover-glass, the further must the systems be separated, i. e., the ad- 

 justing 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 ad- 

 justment are given by Mr. Wenham (Carpenter, yth 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 [z. 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 objetive [i.e., in focusing down], the lenses must be brought 



