20 ON INTERFERENCE IN THE MICROSCOPE. 



distance aci between the corresponding points of the aperture 

 images is, as before shewn=sine a./. Now, as sine a=^ for the 



first diffraction pencil, d being the distance between the striae of 

 the object, the above formula becomes 



a 



or as 7 is the same as the magnifying power m : 



BP = md, 



that is to say, the distance between the interference lines in the real 

 objective image is m times greater than the distance of the bright 

 striae in the object. 



The same action produces also, when combined with the direct 

 image, the diffraction image seen at a' . If, on the other hand, the 

 direct image be shut off, and the two diffraction images are allowed to 

 interfere (in a dark field) then the denominator aa' of the above 

 formula becomes doubled ; consequently, the distance between the 

 lines is reduced to half. In like manner, in so far as other diffrac- 

 tion images come into play, a reduction may be effected by suitable 

 exclusion of light to ^, i, &c., of the distance corresponding 

 to the actual distance. The striation thus appears in such 

 circumstances three or four times finer than in the actual object. 

 There is no difficulty whatever in determining beforehand the 

 diffraction effect when the number and arrangement of the 

 operative diffraction images in the focal plane of the objective is- 

 known. 



The method of shutting off aperture .images in the upper 

 focal plane of the objective (the application of which to the study 

 of diffraction phenomena cannot be sufficiently recommended) 

 yields in addition every evidence that can be desired of the forma- 

 tion of images of minute structure through 'interference.'' As. 



