98 THEORY OF THE MICROSCOPE. 



3. FUSION OF THE INTERFERENCE AND DIOPTRIC IMAGES. 



If the optical power of the Microscope is distributed between 

 two images, produced in essentially different ways, of which one 

 represents the denning, the other the resolving power, then the 

 combined effect obviously does not depend merely upon the 

 perfection of each picture, regarded separately, but also upon 

 the exact superposition of the one upon the other. Since the 

 diffracted rays, on account of their inclination to the axis of the 

 Microscope, meet the marginal parts of the objective with direct 

 (axial) illumination, while the others, according to the deflexion in 

 the object, occupy, some the middle, others the peripheral parts, it 

 follows that an accurate superposition of both images is only 

 possible if the objective is uniformly free from aberration in the 

 whole extent of its aperture. And similarly with oblique illumina- 

 tion, even to a greater degree, since here also the deflected rays 

 may meet the central part of the objective. If this condition 

 cannot be satisfied, or if the objective has considerable errors of 

 construction, a perfect union of the deflected and non-deflected 

 rays, which issue from the same point, is impossible in the final 

 image. It may then happen that the diffraction image is formed 

 in a different plane from the dioptric one, or may appear displaced 

 laterally with regard to it, and systems of stride, which lie in the 

 same plane in the object, require a different focal adjustment to 

 render them perceptible. 



In addition to these propositions of general importance, a few 

 special observations on particular cases may be of service. It is 

 customary to value the performance of an objective according to 

 its resolving power with oblique illumination; consequently, the 

 optician endeavours to increase the angle of aperture as much as 

 possible, and to adapt the whole construction to particular test- 

 objects. The defect of such a proceeding is at once evident. For, 

 obviously, the limit of differentiating power for a given objective 

 lies in such details as allow of at least the first diffracted pencil being 

 admitted under the most favourable circumstances, as well as the 

 direct rays. The two effective pencils touch the margin of the 

 objective in two diametrically opposite points : the direct pencil on 

 the side opposite to the mirror ; the diffraction pencil on the other 



