154 



On the Method of Using Abbe's Test-plate. 



By Dr. C. Zeiss, of Jena. 

 (Communicated ly J. E. Ingpen, September 22, 1882.) 



The test-plate is exclusively designed for the examination of ob- 

 jectives with reference to the correction of spherical and chromatic 

 aberration, and for estimating the thickness of cover-glass for which 

 the spherical aberration is most efficiently corrected. The test-plate 

 consists of a series of six cover glasses, silvered on their under sur- 

 faces, and ranging in thickness from 0*09 mm. to 0"24 mm., 

 cemented side by side on a slide. The thickness of each cover is 

 written on the silver coating. 



Groups of parallel lines are cut through the silver ; these are so 

 coarsely ruled that they may easily be resolved by the lowest powers ; 

 but, from the extreme thinness of the silvering, their contours afford 

 a very delicate test for the most powerful and wide-apertured 

 objectives. 



To examine an objective of large aperture, the plates are to be 

 focussed in succession, observing each time the quality of the image 

 in the centre of the field, and the variation produced by using 

 alternately central and the most oblique illumination. When per- 

 fect correction for spherical aberration exists for the cover-glass 

 thickness of the plate under examination, the contours of the lines 

 in the centre of the field appear perfectly sharp by oblique illumina- 

 tion, without nebulous doubling or indistinctness of the fine 

 irregularities of the edges. If after exactly adjusting the objective 

 for oblique light the illumination is made central, no alteration of 

 the adjustment should be necessary to show the contours with equal 

 sharpness. 



If an objective fulfils these conditions with any one of the plates, 

 it is free from spherical aberration when used with cover-glasses of 

 that thickness ; on the other hand, if every plate shows nebulous 

 doublings, or a confused indistinct appearance of the edges of the 

 silver lines with oblique illumination, or if the objective requires a 



