CH. 



TEST PLATE AND APERTOMETER 



187 



trustworthy results. Differences in the thickness of cover-glasses of o.oi or 

 0.02 mm. can be recognix.ed with objectives of 2 or 3 mm. focus. 



" With oblique illumination the light must always be thrown perpendic- 

 ularly to the direction of the lines." 



FIG. 145. The Abbe Test Plate, lines covered by cover-glasses ranging in 

 thickness from 0.09 to 0.24 mm. 



"The quality of the image outside the axis is not dependent on spherical 

 and chromatic correction in the strict sense of the term. Indistinctness of the 

 contours toward the borders of the field of view arises as a rule, from unequal 

 magnification of the different zones of the objective; color bands in the 

 peripheral portion (with good color correction in the middle) are caused by 

 unequal magnification of the different colored images." 



" Imperfections of this kind, improperly called " curvature of the field," 

 are shown to a greater or less extent in the best objectives, where the aperture 

 is considerable." 



FIG. 146. Abbe Apertometer. 



\ 259. Determination of the Aperture of Objectives with an Apertometer. 

 Excellent directions for using the Abbe Apertometer may be found in the Jour. 

 Roy. Micr. Soc., 1878, p. 19, and 1880, p. 20; in Dippel, Zimmerman, Czapski 

 and Spitta, Ch. XIV. The following directions are but slightly modified from 

 Carpenter-Dallinger, pp. 394-396. The Abbe apertometer involves the same 

 principle as that of Tolles, but it is carried out in a simpler manner; it is 

 shown in Fig. 146. As seen by this figure it consists of a semi-circular plate 

 of glass. Along the straight edge or chord the glass is beveled at 45, and 

 near this straight edge is a small, perforated circle, the perforation being in 



