ABBE'S TEST-PLATE 387 



the field, and thus permit each part of the central or peripheral zones 

 to be brought into play. Against the accurate value of this, ayain. 

 the spherical aberration of an uncorrected condenser would strongly 

 operate. 



Abbe's Test-plate. This test-plate is intended for the examina- 

 tion 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 collected. 



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

 thickness from O09 mm. to 0'24 mm., silvered on the under surface 

 and cemented side by side on a slide, the 

 thickness of each being marked on the silver 

 film. Groups of parallel lines are cut through 

 the films, and these are so coarsely ruled that 

 they are easily resolved by the lowest powers ; 

 yet from the extreme thinness of the silver 

 they also form a very delicate test for objectives 

 of even the highest power and widest aperture. 

 The test-plate in its natural size is seen in fig. 

 331, and one of the circles enlarged is seen in FIG. 332. 



fie- 



. 



To examine an objective of large aperture, the discs must be 

 tbcussed in succession, observing in each case the quality of the 

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

 using alternately central and very oblique illumination. 



When the objective is perfectly corrected for spherical aberration 

 for the particular thickness of cover-glass under examination, the 

 outlines of the lines in the centre of the field will be perfectly 

 sharp by oblique illumination, and without any nebulous doubling 

 01- indistinctness of the minute irregularities of the edges. If, after 

 exactly adjusting the objective for oblique light, central illumination 

 is used, no alteration of the focus should be necessary to show the 

 outlines with equal sharpness. 



If an objective fulfils these conditions with any one of the discs 

 it is free from spherical aberration when used with cover-glasses <>t 

 that thickness. On the other hand, if every disc shows nebulous 

 doubling, or an indistinct appearance of the edges of the lines with 

 oblique illumination, or if the objective requires a different focal ad- 

 justment to get equal sharpness with cential as with oblique light. 

 then the spherical correction of the objective is more or less im- 

 perfect. 



Nebulous doubling with oblique illumination indicates over- 

 correction of the marginal zone ; indistinctness of the edges without 

 marked nebulosity indicates under-correction of this /one ; an 

 alteration of the focus for oblique and central illumination (that is. 

 a difference of plane between the image in the peripheral and central 

 portions of the objective) points to an absence of concurrent action 

 of the separate zones, which may be due to either an average under- 

 or over-correction, or to irregularity in the convergence of the rays. 



The test of chromatic correction is based on the character of the 

 colour-bands which are visible by oblique illumination. With good 



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