1898.] MICROSCOPICAL JOURNAL. Ill 



such methods ; all else is " blurred." Dr. Dallinger had 

 to do such work with a high degree of heterogeneous il- 

 luminatiou — as close an approach as is possible with the 

 lens used, to a self-luminous condition of the object. 



The image even of the diatom is a false image. It is 

 admittedly so in regard to the " spherules," and compe- 

 tent judges are very doubtful whether even the breaking 

 up of the striae so shown, is not due to false diffraction- 

 fringes from the midrib of the valve, the spherules being 

 thus arranged in longitudinal rows far more straight 

 than is really the case. Looking at the matter theoret- 

 ically, it will be observed that after having laid down 

 how the excellence of the image is in inverse proportion 

 to " the degree in which [the state of the light by which 

 the object is illuminated] contributes to produce, to 

 modify, or to efface detail," he proceeds to obtain this 

 image by almost the greatest specialization of the light 

 which is possible. The effect of this is to replace the 

 actual detail, by other apparent detail which is visually 

 intense, and geometrically symmetrical, to an utterly 

 false degree. 



Similar results are traceable in other diatom work by 

 the Abbe school, as may be shown by the most familiar 

 test-valve of all, a much coarser one, the P. angulatum. 

 Dr. Van Heurck has photographed this with the cele- 

 brated Abbe-Zeiss lens of 163 aperture and dense immer- 

 sion-fluid and medium, by Abbe methods, with an un- 

 corrected condenser ; the result is a series of hexagons 

 resembling a honey-comb. Dry objectives can only 

 image details " correctly so far as regards their number 

 and position, but any further detail is not correctly 

 represented." Immersions embracing most of the first 

 spectra, " we now see some detail : the dots appear hexag- 

 onal, and are separated from one another by walls 

 which are thin, and which look like a honey-comb ; " 

 ■end •' this is the first and only step we can take towards 



