138 



Animal Micro log y 



doublet (Fig. 54) or a triplet, each made of different kinds and 

 shapes of glass. A good objective is a very delicate piece of 

 apparatus and must be handled with great care. Each component 

 is very accurately ground and the systems distanced with extreme 

 precision in order to get a clear image. If not already familiar 



3-inch Objective. J-inch Objective. T yinch Oil Immersion Objective. 



FIG. 49. Lens Systems of Various Objectives. 

 Bausch and Lomb 5-inch, J-inch, and ^-inch oil-immersion objectives respectively. 



with the parts of the compound microscope, the student should 

 study Figs. 51 and 52 with a microscope before him. 



DEFECTS IN THE IMAGE 



Spherical Aberration. A simple convex lens, unless corrected, 

 will not give a sharply denned image because it does not refract 

 to the same degree all rays passing through it. Those which 

 traverse its edges are brought to a focus nearer the lens (Fig. 50). 

 This results not only in an indis- 

 tinct image but in a distortion of 

 shape as well. Straight lines, for 

 example, appear curved and when 

 the parts of the object are in focus 

 in the center of the field, those 

 nearer the margin are hazy and 

 indistinct. This defect is greatest in strongly curved lenses, that 

 is, in high powers, since magnification increases with increased 

 curvature. Spherical aberration is corrected by one or more of 

 the following processes: 



