116 THE AMERICAN MONTHLY ~~ [Jun 
all the visible features in the former case, as compared 
with the misleading and unreliable, although well-mark- 
ed and obtrusive, diffraction outlines and etpee invari- 
ably associated with the latter, 
(2) The strong prejudice evinced by many workers in 
favor of an evenly-lighted ‘moon’ field in place of a sharp- 
ly-focussed image of the light source. Itis true that with 
a low-power substage condenser the focussed image of the 
lamp flame can be made to fill the entire field, but the 
rough-and-ready worker finds it much simpler to secure 
this result by lowering or raising his condenser. 
(3) Carelessness and apathy regarding microspical 
manipulation; it being considered that the labor requi- 
site for the acquirement of even an elementary knowledge © 
.of the subject would be a mere waste of time on the part 
of a naturalist or kindred worker. 
(4) A conscientious objection to the critical large cone 
image on theoretical grounds ; for, according to the Abbe 
dictum, ‘‘Strictly similar images cannot be expected, ex- 
cept with a central illumination with a narrow incident 
pencil, because this is the necessary condition for the 
possible admission of the whole of the diffracted light.”’ 
With regard to the first, nothing but practice and pa- 
tient work will enable the microscopist to perceive in a 
delicate object all the minute faintly-outlined features 
just within the resolving power of the objective employ- 
ed, and revealed by it under the action of a solid five- 
sixths or even six-sevenths axial cone from a well-cor- 
rected aplanatic substage condenser. It is true that un- 
der such conditions diffraction effects are conspicuously 
absent, the picture of a translucent object, such as a dia- | 
tom, exhibiting almost as little contrast as would a per- 
forated plate of clear glass held at arm’s length between 
the eye and a bright background. But, although the re- 
sultant image is pale, the delineating and defining power 
of a good lens thus used, is such that in the case of a 4- 
