PHOTOMICROGRAPHY AND TECHNICAL MICROSCOPY 



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plane on either side and in such a way that the strip tapers gradually in thickness from 

 about 0.1 mm. at one end to about 0.2 mm. at the other end. The cover glass with 

 the film side down is permanently mounted on a microscope slide on which is engraved 

 a scale indicating, by increments of 0.01 mm., the thickness of the cover glass from one 

 end to the other. 



The ruled lines are usually designated as the "white spaces" and the undisturl)od 

 silver film between adjacent ruled lines as the "black spaces." The silver film is not 



Fig. 6. — Photograph showing the various parts of Abbe apertometer. 



totally opaque but is sufficiently transparent to reveal the silver grains. The ruled 

 lines have jagged edges which form a delicate test object for high-power objectives. 



To test objectives, a microscope should be used which is fitted with the usual sub- 

 stage illuminating apparatus, in order to provide for a rapid transition from axial to 

 oblique illumination. 



The quality of the image at the center of the field and the changes which occur 

 when the illumination is changed from axial to oblique are carefully observed. The 



A B 



Fig. 7. — Testing numerical aperture of objectives with Abbe apertometer. A shows 

 silver slit of apertometer as viewed through microscope. B shows the back lens of an 

 objective as viewed through the auxiliary microscope. Cross hairs are displaced to right 

 and left. The mean of the readings is the numerical aperture of the objective. 



performance of an objective is judged by the way in which the sharp edges of the 

 rulings and the silver grains themselves are defined and also by the color and width of 

 the color fringes which border the edges of the black spaces. 



Figure 8 illustrates diagrammatically the appearance of color fringes as seen with a 

 test plate and a semiapochromatic objective when using axial light. If the objective 

 is pushed within the focus, the edges appear violet, and if the objective is drawn with- 

 out the focus, the color fringes change to apple green. This is known as "symmet- 

 rical" coloring because both edges of the black spaces are colored the same. 



The cause of these color fringes is made clear in Fig. 9, which illustrates in an 

 exaggerated way the formation of the images with a semiapochromatic objective when 

 using white light and axial illumination of the test plate. In this diagram, the colors 

 are indicated separately instead of blended. 



