156 TESTING THE MICROSCOPE. 



is, the quicker or slower disappearance of the fine details of the 

 image on raising or lowering the body tube, is applicable as. 

 shown by the construction. In most cases, however, unless the 

 aberration of the central rays is considerable, we shall obtain 

 somewhat uncertain results, because the image-forming rays do 

 not, as we have supposed, intersect exactly in the same point, but 

 are partly affected by the opposite aberration. The outlines of the 

 image do not then disappear suddenly either with high or low 

 adjustment, and it is difficult to say which acts more slowly. For 

 this reason we do not enter upon the explanation of other 

 combinations which may occur with respect to the aberrations of 

 the differently inclined rays. The phenomena which have been 

 theoretically determined for either assumption, appear too com- 

 plicated in practice to supply more reliable data for estimating the 

 quality of the objective. 



The practical processes of testing are consequently confined to 

 the following points, summarizing the preceding remarks : 



(1.) We give to the cone of light proceeding from the mirror an 

 aperture at least as large as that of the objective to be examined 

 With high-power objectives this is accomplished by the application 

 of a suitable illuminating apparatus, or by placing an objective- 

 lens of 3 to 4 mm. in diameter under the object-stage. 



(2.) We focus the Microscope to that plane, in which the 

 details of the image (for instance, the edges of fine fissures in a 

 blackened plate) are most distinctly seen. If then a bluish fog is 

 noticeable in the parts surrounding the image, the aberration is 

 not siifficiently corrected. 



(3.) In order to decide which rays of the incident cone are 

 affected by aberration, the illuminating apparatus, or the dia- 

 phragm, must be gradually withdrawn. If the bluish fog rapidly 

 decreases or entirely disappears, the aberration is confined to the 

 marginal rays; if it retains the same intensity for some time, 

 the cause of the phenomenon lies in the central rays. In the 

 latter case, which on the whole occurs less often, the fog should 

 disappear if the central part of the objective is covered by a suit- 

 able stop. 



(4.) We test the phenomena which a higher or lower focal adjust- 

 ment causes. In doubtful cases an alteration of the position of the 

 eye-piece is to be preferred. The over-correction of the objective, 

 with regard to the marginal rays, is hence recognized by the fact, 



