78 



ADJUSTMENT OF OBJECTIVES 



CCh. Ill 



Fig. si. 



Aberration Produced by the 

 Cover-glass. 



from the 



some object, like Pleurosigma (§ 115), whose structure is well agreed 

 upon, and then to practise hghting it, shading the stage and adjusting 

 the objective, until the proper appearance is obtained. The adjust- 

 ment is made by turning a ring or collar which acts on a screw and 



increases or dimiuishes the 

 distance between the sys- 

 tems of lenses, usually 

 the front and the back 

 systems (fig. 35). 



§ 136. Directions for 

 adjustment. — (i) The 

 thicker the cover-glass the 

 closer together are the sys- 

 tems brought by turning 

 the adjusting coUar from 

 the zero mark and con- 

 versely; (2) the thinner 

 the cover-glass, the further 

 must the systems be sep- 

 arated, i.e., the adjusting 

 collar is turned nearer the 

 zero or the mark " un- 

 covered." This also in- 

 creases the magnification of the objective (§ 235). 



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 direc- 

 tions from the first position. Observe the expansion of the dark out- 

 line 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 [i.e., in focusing up], the lenses must 

 be placed further asunder, or toward the mark uncovered [the adjust- 

 ing 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 



Axis The extension of the principal optic 

 axis. 



Cover The cover-glass. 



1,2,3 Three rays originating 

 object mounted in balsam. 



r, r, r Points of refraction as the three 

 rays emerge from the upper surface of the 

 cover into the air. 



Object from which the rays originate. 



1, 2', 3 The three levels from which the rays 

 seem to originate when traced backward from 

 their points of emergence. This gives the 

 effect of spherical aberration (Ch. IX). 



