148 TESTING THE MICROSCOPE. 



3. DEFINING POWER. 



The above-mentioned details of test-objects, which serve as a 

 standard for the resolving power of objectives, belong, as already 

 mentioned, exclusively to the interference image which is pro- 

 duced by the pencils of light diffracted in the object, and 

 reunited in the Microscope. Since these pencils with ordinary 

 illumination pass through the marginal zone of the objective 

 only, they must, of course, be regarded as furnishing unreliable 

 vouchers for the accuracy of its construction in general. It 

 is therefore necessary to test the course of the rays in the 

 objective by other means, which admit chiefly of our judging 

 of the central portion of the objective-aperture. In other words, 

 in addition to testing the structure-image, a careful testing is 

 also necessary of the contour-image which is produced by the 

 direct rays, especially when the middle part of the objective- 

 aperture comes into play. This testing is most effective on 

 objects which show clearly marked contrasts of light and shade 

 in the same plane, or, what is equivalent, sharp outlines on 

 surface-elements that are not too small. Bright spots of different 

 shapes on a dark ground are specially suitable, and they can 

 readily be made upon an object-slide coated with Indian ink 

 or tin-foil. Fragments of diatom- valves having very sharp edges 

 are excellent test-objects for this purpose ; and since, moreover, 

 as Abbe rightly insists, they afford the advantage of testing the 

 structure-image at the same time as its coincidence with the 

 dioptric image, they are unconditionally preferable for a rapid 

 yet sufficiently reliable examination. If these fragments are 

 to satisfy the two-fold purpose, they must first be perfectly flat, 

 and at the same time so thin that outlines and structural 

 details may be regarded as lying in the same plane ; and, secondly, 

 so marked that the interference images may be nearly equal 

 in brightness to those formed by the direct rays, which obtains 

 with dry objects to a higher degree than with those mounted 

 in balsam. 



As regards the choice of objects, the gradations for the lower 

 and medium objectives are easily found ; for the higher powers, 

 however, particularly for immersion objectives, the selection is 

 limited. Fragments of Pleurosigma angulalum may be strongly 



