282 SUMMARY OF CURRENT RESEARCHES RELATINa TO 



of cover-glass under examination, the outlines of the lines in the 

 centre of the field will be perfectly sharp by oblique illumination, and 

 without any nebulous doubling or indistinctness of the minute irregu- 

 larities of the edges. If, after exactly adjusting the objective for 

 oblique light, central illumination is used, no alteration of the focus 

 should be necessary to show the outlines with equal sharpness. 



If an objective fulfils these conditions with any one of the disks, 

 it is free from spherical aberration when used with cover-glasses of 

 that thickness. On the other hand, if every disk shows nebulous 

 doubling, or an indistinct appearance of the edges of the lines, with 

 oblique illumination, or if the objective requires a different focal ad- 

 justment to get equal sharpness with central as with oblique light, 

 then the spherical correction of the objective is more or less im- 

 perfect. 



Nebulous doubling with oblique illumination indicates over- 

 correction of the marginal zone, indistinctness of the edges without 

 marked nebulosity indicates under-correction of this zone ; an alter- 

 ation of the focus 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 

 over-correction or to irregularity in the convergence 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 lines in the centre of the field should show 

 only narrow colour bands in the complementary colours of the secon- 

 dary spectrum, namely on one side yellow-green to apple-green, and 

 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 ef&cient, 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 

 ordinary immersion objectives, the apertures of which are, as a rule, 

 greater than 180° in air (1-00 N.A.), and those homogeneous-immer- 

 sion 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, glycerine, or oil. 

 In ordinary cases the change from central to oblique light may be 

 easily effected by the 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 

 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 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, by which means the sharpness of the outlines and the 



