52 



LIGHTING AND FOCUSING. 



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

 ( Fi g- 57)- 



Fig. 56. — 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. Focus or axial point of the objec- 

 tive. 



F' and F" ' . Points on the axis where 

 rays 2 and 3 appear to originate if 

 traced backward after emerging from 

 f the upper side of the cover-glass ( Cover). 



It is for the correction of this disturbance, or so-called "negative aberration?' 

 produced by the cover-glass that objectives are fixed in their mounting for a given 

 thickness of cover, or that the combinations making up the objective are made ad- 

 justable ; that is, so that the back combinations may be brought nearer the front 

 lens or combination as the cover thickens, and separated with a thinner cover or 

 for uncovered objects. The thicker the cover, the nearer the front and back combi- 

 nations ; the thinner the cover the farther apart are front and back combinations 

 separated (\ 96). 



§ 96. Adjustable Objectives. — The proper adjustment of object- 

 ives, that is, the adjustment which gives the truest image, requires both 

 insight and experience ; for the structure of an object does not appear 

 the same with different adjustments of the objective. And as the opin- 

 ion of different observers on the structure of objects varies, they adjust 

 the objectives differently, and try to obtain the adjustment which will 

 show a structure in accordance with their opinion. Eyes also differ, and 

 two observers might find it necessary to adjust the same objective dif- 

 ferently to produce an identical appearance for each of them. 



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 sys- 

 tems (Fig. 40). In adjustable objectives the back systems should be mov- 

 able, 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. 



General Directions. — (A) The thinner the cover-glass the further 

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



