DR. C. ZEISS ON THE METHOD OF USING ABBE's TEST-PLATE. 155 



different adjustment to get equal sharpness with central as with 

 oblique light, then the spherical corrections are more or less im- 

 perfect. 



Nebulous doubling with oblique illumination indicates over- 

 correction of the marginal zone ; want of sharpness of the edges, 

 without marked nebulosity, indicates under-correction of this zone ; 

 the alteration of the adjustment for oblique and central illumination 

 — that is, difference of level between the image in the peripheral and 

 central portions of the lens system — points to insufficient concurrence 

 of the separate zones, which maybe due to either an average under or 

 over-correction, or to irregularity in the conveyance of the rays. 



The test of chromatic correction is based on the character of the 

 colour bands which are visible by oblique illumination. With good 

 correction the edges of the silver lines in the centre of the field 

 should show but narrow colour bands in the complimentary colours 

 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 colour 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 vertically 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 larger than 180° in air, and homogeneous immersion ob- 

 jectives 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, glycerine, 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 altera- 

 tion of the direction of the mirror. For the examination of objec- 

 tives of smaller angular aperture (under 40° — 50°), we may obtain 

 all the necessary data for the estimation of the spherical and 

 chromatic corrections by placing the concave mirror so far to one 

 side that one edge of it is about in the line of the axis, and there- 

 fore that the incident cone of rays only fills one half of the aperture 

 of the objective, when the sharpness of the contours and the cha- 



