ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
817 
is equivalent to an increase of the aperture, e. g. from 1-40 to 1*75, so 
that here we have a very considerable advance by very simple means. 
Photography, as was first shown by Helmholtz, affords a means by 
which the capacity of the Microscope may be increased. The result, 
however, has not always corresponded to the theory. An important 
point indispensable for practical success has been often overlooked. It 
is in all theoretical deductions tacitly or even expressly assumed that 
the objective used for the photograph will give with the rays of shorter 
wave-length equally good images as with ordinary white light. This 
is, however, by no means the case. In fact, with the objectives of the 
ordinary type, such as alone existed a few years ago, such a result could 
not be attained. If the objective gave good images, i. e. was corrected 
for light of the wave length 0*55 /x, the images from light of wave- 
length 0*44 fx were so bad as to annul the theoretical advantage of the 
increased resolving power. The method employed to obviate this diffi- 
culty was not very successful. It consisted in spherically correcting 
for rays of that wave-length, e. g. A. = 0 • 44, which was most effective 
in the photographic process, and in effecting the chromatic correction so 
that the image corresponding to the wave-length 0 • 55 should coincide 
with the photographically effective image. Thus the latter could be 
correctly adjusted by the naked eye, but the defects remained that 
(1) the optically effective image was in itself bad (spherically under- 
and chromatically over-corrected, and (2) in the photo-chemically 
effective parts of the spectrum the concentration of the light was very 
incomplete, so that owing to the under-correction of this part of the 
spectrum there was danger of one part obscuring the image produced by 
the other. 
The apochromatics have rendered the greatest service in this direc- 
tion. In fact the advantage of their use in photomicrography is even 
more pronounced than their recognized superiority in ordinary micro- 
scopic work. This is due to the fact that with these objectives the 
images corresponding to the different wave-lengths right up to the violet 
are practically coincident in position and magnitude. Since their intro- 
duction cases have continually multiplied in which structures have been 
made visible by photography which could not be resolved by other 
means. But even with the apochromatic the conditions have not always 
been kept upon which an advance in the capacity of the objective 
depends. The author considers that such an advance by means of 
photography depends upon tbe following conditions : — 
The system employed should be suitably corrected, so that the images 
resulting from the short wave-lengths may be sharply defined and coin- 
cident in position with that which affects the eye. The second condition 
is that the light of the required short wave-length should be photo- 
graphically effective. This requires that (1) the source of light must 
emit waves of the required shortness, and these with sufficient intensity ; 
(2) the rays corresponding to the larger wave-lengths must be excluded 
in such a way that the intensity of the short-wave rays shall not be too 
much reduced ; (3) the photographic plate must be sufficiently sensitive 
for the light of the required wave-length ; (4) all media between source 
of light and photographic plate must transmit the rays of the required 
short wave-length. This last requirement draws the limits to the 
