591 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
as so often described, will be seen. They interfere, and form tbe image 
seen by the eye-piece in tbe Fresnel and Abbe manner. Removing the 
stage-grating, and replacing the coarse one over the flame, its focal image 
is now the object. Owing to the heterogeneity of the rays, this aerial 
image emits no spectra — there neither arc nor can be any such. But it 
is perfectly resolved. Here we have a resolution of 3000 or 6000 lines 
per inch that has no place at all in the “spectrum” theory; which, 
therefore, can be no complete theory of microscopic vision, though it has 
an important place in it. 
It is highly desirable to find out what proportion and value must be 
assigned to the Abbe image in ordinary research. There arc really two 
factors in the standard image, which is the outcome partly of the features 
upon the object, and partly of the state of the light by which the object 
is illuminated ; and Dr. Stoney himself says, “ It (the image) may be im- 
proved by increasing the degree in which the first of these factors, and 
by decreasing the degree in which the second, contributes to produce, to 
modify, or to efface detail in the image.” With this statement the author 
cordially agrees, but differs from Dr. Stoney in the estimate of the true 
relative proportions of the factors. Thus the results of diatom photo- 
micrography on Abbe methods are wofslly inferior. 
Wherever we have a known periodic structure in transparent objects, 
plane wave illumination and the consequent interference lines formed 
by the beams diffracted by that structure have an extraordinary effect in 
intensifying into black and white a more or less accurate representation 
of the periodic detail. (Two experiments illustrating this are described.) 
Thus the Abbe method has a most important function in enabling us to 
see contrast in the details of a large class of objects — especially hyaline 
or transparent objects — which do not present contrast or opacity suffi- 
cient to be seen in any other way. The error has been in giving to 
it the sole or all-important place, not recognising that there is quite 
another kind of image also available, depending upon Airy’s theory ; and 
that the latter — while in the case of transparent details often giving 
images insufficient, or at least far inferior, in black-and-white contrast — 
is free from the contour errors of the Abbe image, and must be used to 
correct it so far as is possible in the individual cases. The errors of the 
“ spectrum ” image are well known. Its very contrast or “ resolution ” is 
a departure from truth, to which the more indistinct self-luminous image 
is in reality a much nearer approach. It tends to make details, which 
should be only geometrically symmetrical to a limited extent, perfectly 
so. All that can be really learnt from it is that there is probably some 
periodic difference of structure in the object similar in dimensional in- 
tervals to “lines” shown; less probably so in regard to “ spots,” since 
these are often produced by false diffraction fringes from any long line 
which may cross the true ones. That the lines are lines, or that the 
“pattern” is so geometrical as appears, is in the highest degree im- 
probable. That the “ spectrum ” theory and method so long retained 
exclusive predominance is because attention has been so concentrated 
upon either gratings or diatoms of known periodicity in structure, but 
which only represent to a very small extent indeed any serious kind of 
investigation. 
It thus appears that in Microscopy we have to deal with two charac- 
