CH. X} 



TEST PLATE AND APERTOMETER 



269 



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 illuminat- 

 ing 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 illluminator 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 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 the examination of objectives of smaller aperture (less than 4o-5o) 

 we may obtain all the necessary data for the 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 noticeable 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 trust- 

 worthy results. Differences in the thickness of cover glasses of o.oi 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. 



FIG. 215. The Abbe Test Plate. 



"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 



