186 TEST PLATE AND APERTOMETER \_CH. VII 



"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- 

 undercorrection 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 con- 

 current 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 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-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 objective and a rapid interchange from oblique to central light Abbe's 

 illuminating apparatus is very 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, glycerin 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 impossible 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°-50°) 

 we may obtain all the necessary data for the estimation of the spherical and 

 chromatic 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 notice- 

 able with such objectives." 



" 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 



