ON INTERFERENCE IN THE MICROSCOPE. 21 



soon as the aperture images occasioned by diffraction are shut off, 

 so that on looking down the tube the direct image only is visible, the 

 image of the object appears entirely void of fine details, — every 

 delineation of minute structure has disappeared just as if the instru- 

 ment had suddenly lost its optical power. But immediately a single 

 diffraction image is admitted into play as a second source of light, 

 the striation appears very clearly in that direction which is perpen- 

 dicular to the line joining the two operative pencils, whilst in 

 directions parallel to this line of junction all striation is as absent 

 as it was before. If, for example^ such an object as Pleurosigma 

 attenuatum be viewed through an objective of |th-inch focal length 

 and say 1 80° aperture, there will be seen, beside the direct aperture 

 image, four diffraction images, of which each two diametrically 

 opposed images correspond to a single system of striae : according 

 as one or the other pair be shut off the transverse or longitudinal 

 striations in the field disappear ; and if all four diffraction images 

 be shut off no striation of any kind will be seen. 



In the foregoing observations homogeneous light has for the 

 sake of simplicity been assumed throughout, and the wave length 

 taken at o.j mik. As a rule, however, the various coloured rays 

 operate together -, and as the separation of the lines in the 

 diffraction images is proportional to the several wave lengths, the 

 variously coloured images appear laterally displaced, and form a 

 regular spectrum, in which even Frauenhofer lines appear when 

 the illumination is favourable. As, however, all diffraction 

 images which come from the same elements in the object 

 cross each other at the same points in the focal plane of the 

 objective image, — in so far as chromatic and spherical aberration 

 are not taken into account — they, therefore, produce an interference 

 image sufiSciently defined and colour free. The limit of distin- 

 guishability will, from the very fact that each colour i$ concerned 

 in the image, generally be determined by the rays of mean 

 refrangibility (whose wave length amounts to about o.j mik.) In 

 particular cases it may, however, happen, that rays of greater 

 refrangibility {e.g., green or blue) preponderate, and in photographs 



