ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
593 
microscopic objective of focus 1/50 in., the light would travel a path of 
perhaps 1/200 in. from the object to the lens, and such light canuot be 
regarded as consisting of uniform plane waves, except in the case of 
plane-wave illumination of the object, as in the Abbe theory. 
In discussing the supposed dynamical system, Mr. Wright admits 
that a reversal of the ivliole actual system would produce such an image 
as described ; but it does not seem to follow that mere “ coalescence and 
interference of uniform plane waves ” involves such a result. In any 
case, what the reversal of the supposed dynamical system must really re- 
produce as an image at the plane of its origin, must be the postulated 
operative cause of the system. That cause, by the hypothesis, is not an 
actual object which alone emits luminous waves, but the object sur- 
rounded by an indefinite number of identical replicas , emitting identically 
similar plane waves. This does not represent any object in reality; 
and that fact seems to dispose of such a presentment as a full and 
complete representation of microscopic vision. 
In considering how far the Abbe theory, which possesses more or less 
undoubted truth, is an adequate representation of microscopic vision, the 
author states his own opinion thus : — “ The trustworthiness of a microscopic 
image is in proportion as the object approximates to a self-luminous con- 
dition, and diminishes in proportion as it is or has to be (for it may have 
to be) examined by plane- wave illumination.” 
If the object be self-luminous there will be no spectra, and the rays 
emitted will be quite heterogeneous ; yet an image must be possible. 
Really self-luminous objects can hardly be used with objectives above an 
inch, but approximations to self-luminosity can be obtained in various 
ways with high powers. Thus Lord Rayleigh has shown that the wide 
cone from a condenser introduces a large amount of heterogeneity into 
the rays from the objects, and practically produces self-luminosity. 
Dr. Stoney rather seems to regard the function of the condenser as that 
of providing illumination by plane waves. Thus, according to Dr. Stoney, 
the ideal is to get absolutely aplanatic systems of plane waves trans- 
mitted through the objects, and all conditions short of this impair the 
image ; but according to the author, irregularities of phase add to the 
trustworthiness of the image, though they may impair it in some other 
features. A very important practical question is therefore at stake. 
The following experiment is one of those by which the author proves 
his view. He takes as his stage object a grating of 3000 or 6000 lines 
to an inch, illuminated by a narrow cone from the condenser, focussing 
the flat of a rather distant lamp-flame. Immediately in front of this 
flame he places a coarse grating of 50 to 100 lines per inch, either photo- 
graphed or of wire. The several points of these luminous lines emit 
light-waves, chiefly in the self-luminous manner, indiscriminate in 
phases and transversals as the points of the flame itself. Arranging the 
stage grating so as to cover only half the objective field, a condenser can 
be selected of such a focal length, aud 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, 
the same illuminating cone being used. R move now the coarse grating, 
and place the stage grating centrally ; then, on removing the eye-piece, 
and looking down the tube, the dioptric beam or its flanking spectra, 
