CHAPTER XII 

 THE OBJECTIVE 



Structure. — The objective is made of single lenses 

 (for example, the front lenses of the higher powers), as well 

 as doublets, and sometimes triplets, whose function is to 

 take as much as possible of the divergent Hght from the 

 object and change it into the sHghtly convergent rays that 

 meet in the upper end of the microscope tube in the front 

 focal plane of the eyepiece, forming a magnified image there, 

 to be further magnified by the eyelens. It is obvious (from 



the equation = 4-^, where is the distance of the object 



from the front focal plane of the objective, /i and/o the front 

 and back focal lengths of the objective, and L is the optical 

 tube length of the microscope) that the longer the micro- 

 scope tube, the nearer the front lens of the objective must be 

 to the object; that is, the shorter is the working (or viewing) 

 distance (though not in proportion). This working dis- 

 tance is also shortened by separating the front and back 

 combinations of the objectives, as is done by a correction 

 collar; and also by putting a converging achromatic lens 

 behind the objective. In this last way, a dry objective 

 might be corrected for uncovered objects. On the other 

 hand, an achromatic diverging lens, or amphfier, behind 

 the objective, may be used to correct for too thick a cover- 

 glass, such as the 0.4-millimeter cover used in counts of 

 blood corpuscles. Much of the magnification of a high 

 objective is done by the front lens (or two lenses in the 

 highest powers), which is uncorrected and small, and thus 

 of strong curvature. The back combinations of doublets 

 or triplets serve mainly to supply the corrections. An 

 objective is corrected for one definite tube length only 

 (and one cover-glass thickness) at which it is at its best. 



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