ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 58 # 
thus: — Place the dark slide iu position in the camera, taking care before 
withdrawing the shutter to cut off the light entering the instrument by 
placing a portion of blackened card between the object and the lens. 
Now, instead of pulling the shutter out the whole distance, only pull it 
out 1 inch and expose 3 seconds ; replace the card, draw out the shutter 
another inch and expose G seconds ; again replace the card and draw 
out the shutter 1 inch, exposing 6 seconds; once more repeat the fore- 
going, again giving 3 seconds only. On developing you will find four 
ranges of exposures, the last inch having received 3 seconds, the next 
to it 9 seconds, the next to that 15, and the next, being the first portion 
exposed, 18 seconds. 
“ When we develop this plate we shall be able to tell at a glance 
which part of the plate has had the nearest to the correct exposure, and 
shall know how to act with the next.” 
C5) Microscopical Optics and Manipulation. 
Relation of Aperture to the Determination of Minute Structure.* 
— Mr. C. F. Cox gives vent to some rather heretical remarks concerning 
the Abbe diffraction theory. The central idea of Prof. Abbe’s theory is 
that in order to obtain a true image of an object, all light-rays from the 
object must be gathered up by the objective and recombined at its back. 
The finer the details of structure in the object the greater the dispersion, 
so that in order to admit all the rays, the aperture of the objective must 
be proportionately widened. Our finest optical combinations, however, 
fall far short of an ideal performance in this respect, and, according 
to Prof. Abbe, no known method of construction can produce an 
objective capable of giving a true image of details of structure which 
are closer to each other than within 1/2500 in. It is also the teaching 
of Prof. Abbe that strictly similar images cannot be expected, except 
with central illumination with a narrow incident pencil. 
The author asks why, if the above statements are true, microscopists 
continue to use liigh-power objectives or buy wide-angled substage 
condensers ? He considers that the reason is that they do not really 
believe the statements ; and though they go on repeating the orthodox 
creed, they qualify it by some secret mental reservation. He confesses 
that he himself is one of the heretics who still places more or less 
reliance upon what is seen through the modern high-power objective. 
Referring to Prof. Abbe’s experiments to prove that the diffraction- 
spectra are necessary to the formation of the image, he points out that 
a progressive reduction of aperture should result in wider and wider 
divergence from faithfulness in the image, and states that experience 
does not confirm this inference. In proof of this point he refers to a set 
of photographs of the Abbe diffraction-plate made by Mr. T. F. Smith, 
showing the effect of using successively a stop with a 1/1 2-in., a 1/20-in., 
and a 1/32-in. opening. Other photographs by Mr. Smith of diatom 
valve, 3 showing the fibrillar structure are also referred to. Photographs 
of the same diatom valves taken first with an old-time water-immersion 
1/12, amd then with a recent oil-immersion apochromatic 1/12, were 
wholly ip favour of the wider-angled glass, but the author considers that 
* Journ. New York Micr. Soc., xi. (1895) pp. 74-85. 
