82 



We supply two different patterns of the apertometer; in the form (No. 155) 

 shown in Fig. 40 the indicators move upon the glass disc direct, whereas in the 



View from above. 



Fig. 40. 

 Apertometer, No. 155. 



(Full Size.) 



Side view. 



second model (No. 156) the glass disc is mounted upon a metal plate with a 

 slot for the more certain and more convenient adjustment of the indicators. 



No. 157. Test-plate. For testing lenses with respect to spherical and 

 chromatic aberrations and for estimating the thickness of the cover-glass, which 

 corresponds to the most perfect correction, a test-plate was designed by ABBE 

 in 1873. Used in conjunction with the ABBE Illuminating Apparatus and by 

 establishing the sensitive rays, it gives the greatest prominence to any existing 

 faults of correction. 



This test-plate consists of an object carrier with six cover-glasses of accur- 

 ately determined thickness (0.09 mm to 0.24 mm) cemented side by side. These 

 cover-glasses are silvered on their lower surfaces and engraved with lines, the 

 jagged edges of which form the test-object proper. 



* 



See E. ABBE in Archiv f. mikr. Anat, vol. IX, 1873, pp. 434437, Ge- 

 sammelte Abhandl. I, 1904, pp. 6668, also DIPPEL, "Das Mikroskop". vol. I, 

 p. 340, and the directions accompanying the test-plate. 



