APPENDIX.] ABBE'S TEST-PLATE. 211 



green to apple green on the other violet to rose. The more perfect the correction 

 of the spherical aberration the clearer this color band appears." 



"To obtain obliquity of illumination extending to the marginal zone of the ob- 

 jective and a rapid interchange from oblique to central light Abbe's Illuminating 

 apparatus is ver\' efficient, as it is only necessary to move the diaphragm in use 

 nearer to or further from the axis by the rack and pinion provided for the purpose. 

 For the examination of immersion objectives, whose aperture as a rule is greater 

 than 180 in air and those homogeneous-immersion objectives, which considerably 

 exceed this, it will be necessary to bring the under surface of the Test-plate into 

 contact with the upper lens of the illuminator by means of a drop of water, gly- 

 cerin or oil." 



" In this case the change from central to oblique light may be easily effected by 

 the ordinary concave mirror but with immersion lenses of large aperture it is im- 

 possible to reach the marginal zone by this method, and the best effect has to be 

 searched for after each alteration of the direction of the mirror." 



" For the examination of objectives of smaller aperture (less than 4o°-5o°) we 

 may obtain all the necessary data for the estimation of the spherical and chro- 

 matic corrections by placing the concave mirror so far laterally, that its edge is 

 nearly in the line of the optic axis the incident cone of rays then only filling one- 

 half of the aperture of the objective. The sharpness of the contours and the 

 character of the color bands can be easily estimated. Differences in the thickness 

 of the cover-glass within the ordinary limits are scarcely noticeable with such ob- 

 jectives." 



" It is of fundamental importance in employing the test as above described to 

 have brilliant illumination and to use an eye-piece of high power." 



" When from practice the eye has learnt to recognize the finer differences in the 

 quality of the contour images this method of investigation gives very trustworthy 

 results. Differences in the thickness of cover-glasses of 0.01 or 0.02 mm. can be 

 recognized with objectives of 2 or 3 mm. focus." 



" With oblique illumination the light must always be thrown perpendicularly to 

 the direction of the lines." 



" The quality of the image outside the axis is not dependent on spherical and 

 chromatic correction in the strict sense of the term. Indistinctness of the con- 

 tours towards the borders of the field of view arises as a rule, from unequal mag- 

 nification of the different zones of the objective ; color bands in the peripheral 

 portion (with good color correction in the middle) are caused by unequal magnifica- 

 tion of the different colored images." 



"Imperfections of this kind, improperly called 'curvature of the field,' are 

 shown to a greater or less extent in the best objectives, where the aperture is con- 

 siderable." 



Fig. 164. Set of lines under one cover-glass in the Abbe Test-Plate. 



