TESTING THE SPHERICAL ABERRATION. 149 



recommended. According to Abbe, this object may be advan- 

 tageously used even with the highest powers, if fragments of 

 delicate specimens are selected, and if the eye is directed 

 specially to note the appearance of the image near the margin. 

 For testing the highest dry objectives, coarser specimens of the 

 same object, also in fragments, may be used, although the 

 markings are rather too fine for an angular aperture of, say, 100. 



With regard to the method of illumination to be employed in 

 the testing, Abbe recommends principally two fixed positions of 

 the mirror. First, that it should be placed vertically to a series 

 of lines on the object, so that one edge is approximately in 

 the axis of the Microscope ; the direct rays thus form a slightly 

 inclined cone of light, which grazes the centre of the objective, 

 and consequently occupies the intermediate portion of the 

 aperture ; the diffracted rays, however, pass through the opposite 

 marginal zone of the objective, and produce with the direct 

 rays a structure-image dependent on this zone. Secondly, to 

 move it laterally to produce the most oblique illumination. 

 In this case the two cones of light change places ; the direct 

 one passes through the periphery, and the diffracted one through 

 the centre of the objective. 



With an accurately constructed objective, the contour-image, 

 in both cases, should not only appear equally sharp through- 

 out, but should coincide with the structure-image, without 

 difference of level and without lateral displacement. If an 

 objective fulfils these requirements, at least in the centre of 

 the field of view, it may be depended upon for producing 

 accurate images. An intermediate position of the mirror would 

 furnish additional proof that the different zones co-operate 

 simultaneously in the production of the images. 



II. 



TESTING THE SPHERICAL ABERRATION. 



HAVING explained the methods of testing the optical power of 

 the Microscope in general, we purpose examining the different 

 elements or factors which determine that power. Among these 

 factors, according to our experience, spherical aberration is the 



