SCIENTIFIC SUMMARY. 
325 
Angular Aperture ; its Uses. — 11 The Academy ” (May 8) says that this 
■question, which is of great interest to microscopists, has just been brought 
before the Royal Microscopical Society by Mr. Slack, who contends that 
the extreme angles of aperture usually given to the higher objectives are 
bad substitutes for better correction of spherical aberration. In proof of 
this opinion, he showed that a glass by Zeiss of Jena, with an angle of 
•only 68°, would display the transverse ribs of Surirella gemma divided into 
beads, when the object was illuminated by Mr. Wenham’s dark-ground re- 
flex apparatus. C and D eye-pieces were employed for this purpose, and the 
beads were quite distinct, though it was not pretended that they were as well 
shown as they could be with a higher power and larger angle. Zeiss’s half- 
inch, the C of his catalogue, with 48° aperture, suffices to show the cross- 
beading of Plerosigma hippocampus with B and C eye-pieces and an achro- 
matic condenser. A paper of Professor Abbe was quoted, alleging reasons 
why no dry objectives should have apertures of more than 105° to 110°, 
and why it was well to restrict immersion objectives to little more than 100°, 
so that 55 could work well through covering glass a fifth of a millimetre in 
thickness. Objectives by Zeiss upon Professor Abbe’s plan were found to 
unite in a remarkable degree the qualities of penetration and resolution. 
Action of Cobra-poison on Vegetable Protoplasm. — The following very in- 
teresting experiments were made by Mr. Charles Darwin, and were published 
in a paper read before the Royal Society by Drs. Brunton and Fayer. Mr. 
Darwin says : “ You will perhaps like to hear how it acted on Prosera. 
I made a solution of \ gr. to 5\j of water. A minute drop on a small pin’s 
head acted powerfully on several glands, more powerfully than the fresh 
poison from an adder’s fang. I also immersed three leaves in 90 minims of 
the solution ; the tentacles soon became inflated and the glands quite white, 
as if they had been placed in boiling water. I felt sure that the leaves were 
killed ; but after eight hours’ immersion they were placed in water, and 
after about forty-eight hours re-expanded, showing that they were by no 
means killed. The most surprising circumstance is, that, after an immersion 
of forty-eight hours, the protoplasm in the cells was in unusually active 
movement. Now, can you inform me whether this poison, if diluted, arrests 
the movement of vibratile cilia P I dissolved \ gr. [of cobra-poison] in 5j 
of water, so that I was able to imrqerse two leaves. It acted as before, but 
more energetically ; and I observed more clearly, this time, that the solution 
makes the secretion round the glands cloudy, which I have never before 
observed. But here comes the remarkable point ; after an immersion of 
forty-eight hours, the protoplasm within the cells incessantly changes form, 
.and I never saw it on any other occasion so active. Hence I cannot doubt 
that this poison is a stimulant to the protoplasm ; and I shall be very curious 
to find out in your papers whether you have tried its action on the cilia and 
on the colourless corpuscles of the blood. If the poison does arrest their 
movement, it will show that there is a profound difference between the pro- 
toplasm of animals and of this plant. Therefore if you try any further ex- 
periments I hope that you will be so kind as to inform me of the results. 
I may add that I tried at first 1 gr. to the ^j, as that is my standard strength 
for all substances. It is certainly very remarkable that the poison should 
net so differently on the cilia and on the protoplasm of Prosera. After 
