CHAPTER X 



THE ABBE TEST PLATE AND APERTOMETER ; EQUIVA- 

 LENT FOCUS OF OBJECTIVES AND OCULARS ; DRAW- 

 INGS FOR PHOTO-ENGRAVING ; WAX MODELS 



\ 429. On the Method of Using Abbe's Test-Plate. This test-plate is in- 

 tended for the examination 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 corrected. 



"The test-plate consists of a series of cover-glasses ranging in thickness from 

 0.09 mm. to 0.24 mm., silvered on the under surface and cemented side by side on 

 a slide. The thickness of each is written on the silver film. Groups of parallel 

 lines are cut through the film and these are so coarsely ruled that they are easily 

 resolved by the lowest powers, yet from the extreme thinness of the silver they 

 form a very delicate test for objectives of even the highest power and widest 

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

 in succession observing each time the quality of the image in the center 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 contour of the 

 lines in the center of the field will be perfectly sharp by oblique illumination 

 without any nebulous doubling or 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 adjustment should be necessary to show 

 the contours with equal sharpness. ' ' 



"If an objective fulfills 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 doubling or an indistinct appearance of 

 the edges of the silver lines, with oblique illumination, or if the objective requires 

 a different adjustment to get equal sharpness with central as with oblique light, 

 then the spherical correction is more or less imperfect." 



"Nebulous doubling with oblique illumination indicates overcorrection of the 

 marginal zone, want of the edges without marked nebulosity indicates under- 

 correction of this zone ; an alteration of the adjustment for oblique and central 

 illumination, this 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 overcorrection 

 or to irregularity in the convergence of the rays. ' ' 



"The test of chromatic correction is based on the character of the color bands, 

 which are visible by oblique illumination. With good correction the edges of the 



