APPENDIX. 



Abbe's Test-Plate ; The Apertometer ; Experimental Determination of llie 

 Equivalent Focus of the Objective and of the Ocular ; Testing Homogeneous 

 Fluid ; Preparation of Diagrams ; Drawings for Photo-Engraving. 



\ 352. For the sake of those who may have opportunity to use Abbe's Test- 

 Plate the following directions from Zeiss are appended : 



"CARL ZEISS, JENA. ON THE METHOD OF USING ABBE'S TEST-PLATE." 



" This test-plate is intended 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 o 24 mm., silvered 011 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 deljcate 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 illumina- 

 tion is used no alteratiou of the adjustment should be necessary to show the con- 

 tours 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 undercor- 

 rectiou of this zone ; an alteration of the adjustment 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 overcorrection 

 or to irregularity iu 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 

 silver lines in the center of the field should show but narrow color bands in the 

 complementary colors of the secondary spectrum, namely on one side yellow- 



