118 THE AMERICAN MONTHLY [July 



ience the vital necessity in diflBcult research of a wide 

 cone, write expressly of " theory and practice being thus 

 at variance," in some way or other which had to be ex- 

 plained. 



It is difficult to estimate the prejudicial effect of this 

 upon microscopy on the Continent. As a quite uncor- 

 rected condenser will ^ive a fair cone up to 0-50 N. A., 

 amd also by immersion extremely oblique rays from its 

 margin (equivalent to annular marginal illumination), 

 for years no better Continental condenser was made. 

 Prof. Abbe at last was driven to compute an achromatic, 

 but this last production of Continental microscopy only 

 gives an aplanatic cone of 0'65. Except those few who 

 know of English condensers, with their aplanatic cones 

 of I'lO for immersion and 0-90 for dry conbinations 

 Continental workers have thus been condemned to the 

 errors and weaknesses of narrow pencils, which have 

 thence been propagated through our own medical schools 

 and the results are sufficiently striking. Dr. Koch at 

 last found out, empirically, that wide cones gave much 

 sharper and "finer" images of bacteria, in fact the only 

 images worth having. Prof. Abbe accounted for this 

 observational fact, in an article expressly contradicting 

 any advantage whatever to the image (as an image) from 

 a wide cone, on the ground that the wide cone, owing to 

 its more sharply defined focal plane (want of " penetra- 

 tion "), makes invisible the transparent tissues in which 

 the bacteria are situate. But he fails to account for the fact 

 that it is just the same with bacteria in invisible culture- 

 media or sputum ; and that the advantage really consists 

 in the much greater sharpness or thinness of the images 

 of the bacteria themselves; in truth of contour, so that 

 square ends are shown square and not rounded ; and in 

 the fact that there are no blurred edges or diffraction- 

 fringes around them, as appear with a narrow cone. In 

 fact, many allied bacteria cannot be distinguished at all 



