CHAPTER VII 



THE ABBE TEST PLATE AND APERTOMETER ; EQUIVA- 

 LENT FOCUS OF OBJECTIVES AND OCULARS; 

 CLASS DEMONSTRATION IN HISTOLOGY 

 AND EMBRYOLOGY 



APPARATUS AND MATERIAL FOR THIS CHAPTER 



Abbe test-plate ({S 25S); Apertometer (j! 259); Tester for immersion liquid 

 <$ 260); Microscope with 250 mm. tube and objectives ($ 262); Stage microme- 

 ter (g 262); Filar micrometer with positive ocular ({! 262); Oculars ((S264). 



Demonstration microscopes and dissecting microscope (Figs. 147-149); 

 Traveling microscope (Figs. 150-151); Indicator or pointer oculars (Figs. 152- 

 154); Compound microscope (Fig. 155); Projection microscope (Figs. 158-160). 



TEST PLATE AND APERTOMETER 



I 258. On the Method of Using Abbe's Test-Plate. — This test-plate is 

 intended for the examination of objectives with reference to their corrections 

 for spherical and chromatic aberration and for estimating the thickness of the 

 cover-glass for which the spherical aberration is best corrected. 



" The test-plate consists of a series of cover-glasses ranging in thickness 

 from 0.09 mm. to 0.24 mm., silvered on the under surface and cemented side 

 by side on a slide. The thickness of each is written on the silver film. Groups 

 of parallel lines are cut through the film and these are so coarsely ruled that 

 they are easily resolved by the lowest powers, yet from the extreme thinness 

 of the silver they form a very delicate test for objectives of even the highest 

 power and widest aperture. To examine an objective of large aperture the 

 plates are to be focused in succession observing each time the quality of the 

 image in the center of the field and the variation produced by using alter- 

 nately central and very oblique illumination. When the objective is perfectly 

 corrected for spherical aberration for the particular thickness of cover-glass 

 under examination, the contour of the lines in the center of the field will be 

 perfectly sharp by oblique illumination without any nebulous doubling or 

 indistinctness of the minute irregularities of the edges. If after exactly 

 adjusting the objective for oblique light, central illumination is used no alter- 

 ation of the adjustment should be necessary to show the contours with equal 

 sharpness. ' ' 



