248 Extracts from Mr. H. E. Fripps Translation of 
strongly as the real network of the object, and with equal distance 
between the lines, inclined also 60° to the others ; and when the 
three sets are seen together there will be seen perfectly sharply 
defined six-sided spaces (fields) of the kind observed on Pleuro- 
sigma angulatum, instead of the rhombic fields. It may be added 
that all the appearances unconformable with the structure of 
known objects which are here described were observed with exactly 
the same focussing under which the normal image appeared well 
defined, and that they occurred under various combinations of ob- 
jectives and oculars with regular constancy whenever the illumina- 
tion was regulated in the same way. 
The constant increase of resolving power resulting from oblique 
illumination, and the greater prominence of what was before visible 
with central illumination, is in every instance solely produced 
either by the entrance of diffracted rays into the larger aperture 
(with oblique illumination), which would otherwise not have en- 
tered into the objective on account of their greater divergence, or 
because diffraction pencils which were but imperfectly taken up 
when direct illumination was used now enter more completely and 
work with greater effect, whilst the direct rays are relatively less 
operative. 
The facts here recounted appear sufficient, when taken in con- 
nection with incontestible laws of the theory of undulation, to 
warrant a series of most important conclusions which affect the 
doctrine of microscopic vision, as well as the composition and mani- 
pulation of the microscope. 
Firstly, as respects the vision of objects under the microscope. 
Any part of a microscopic preparation which, either from its being 
isolated, or from its relatively large dimensions, produces no per- 
ceptible diffracted effect, is delineated in the field of the microscope 
as an image formed according to the usual dioptric laws of rays 
concentrating in a focal plane. Such an image is entirely negative, 
being dependent on an unequal transmission of light which partial 
absorption of the rays (e. g. coloured rays), or divergence of the 
rays (from refraction), or diffraction of the rays (produced by par- 
ticles of internal structure), severally occasion. The absorption 
image thus produced is an unquestionable similitude of the object 
itself, and if correctly interpreted according to stereometric rules, 
admits of perfectly safe inferences respecting its morphologic con- 
stitution. On the other hand, all minute structures whose elements 
lie so close together as to occasion noticeable diffraction phenomena 
will not be geometrically imaged, that is to say, the image will not 
be formed, point for point, as usually described by the reunion in 
a focal point of pencils of light which, starting from the object, 
undergo various changes of direction in their entrance and passage 
tlirough the objective. 
