1898.] MICROSCOPICAL JOUR:NtAL. 99 



to obviate the sharply-marked interference-bands which 

 arise when permanent and definite phase-relations are 

 permitted to exist between the radiations whicli issue 

 from various points of the object." Since Dr. Stoney, 

 however, seems rather to regard the function of the 

 condenser as being that of providing illumination by 

 plane Avaves, we had better resort to other methods, 

 which may help us to decide what is a very important 

 practical question. For while the ideal is to get abso- 

 lutely aplanatic systems of plane waves transmitted 

 through the object, and all conditions short of this 

 (caused by imperfections in the slide or various other 

 details) impair the image (as in one special sense they do 

 impair it, with some objects); according to the view ex- 

 pressed, irregularities of phase thus produced may add to 

 the trustworthiness of the image, though it may impair it 

 in some other features. 



Take therefore as an object on the stage, a grating of 

 3,000 or 6,000 lines to an inch, illuminated by a narrow 

 cone from the condenser, focussing the flat of a rather 

 distant lamp-flame. Place immediately in front of this 

 flame a coarse grating, 50 to 100 lines per inch, either 

 photographed or of wire. The several points of these 

 luminous lines emit light- waves chiefly in the self- 

 luminous manner, indiscriminate in phases and transver- 

 sals at the points of the flame itself. Arranging the 

 stage grating so as to cover only half the obj«^ctive field, 

 a condenser can be selected of such a focal length, and 

 other matters so adjusted, that the focal image of the 

 coarse grating formed by the condenser, corresponds both 

 in intervals and focal plane with the object-grating on the 

 stage, and using the same illuminating cone. Remove 

 now the coarse grating and place the stage grating cen- 

 trally : then removing the eyepiece and looking down 

 the tube, the dioptric beam and its flanking spectra as so 

 often described will be seen; they are the images of the 



