SCIENTIFIC SUMMARY. 
107 
MICROSCOPY. 
The Vascular System in Oikopleura. — Mr. J. Sanders, F.R.M.S., read a most 
valuable paper before a late meeting of the Royal Microscopical Society, on 
the anatomy of this genus. The paper, with two illustrations, will be 
found in the “ Monthly Microscopical Journal ” for Nov. 1874. With 
regard to the vascular system the author says : “ The only part of the vas- 
cular system that is visible is the heart. The blood being colourless and 
totally devoid of corpuscles, unless some extraneous matter, such as zoo- 
sperms, becomes mixed with it, the vessels which convey it are invisible. 
The heart is situated between the right lobe of the stomach and the second 
part of the intestine. It is composed of several longitudinal fibres which 
are attached anteriorly along a transverse fibre, which, passing immediately 
in front of the right lobe of the stomach, is fixed to the parietes of the body. 
Posteriorly they join together to form a single fibre, which, passing behind 
this lobe, is in like manner attached to the parietes at that point. When 
the generative gland increases in size it pushes between the right lobe of 
the stomach and the first part of the intestine, giving the appearance as if 
the posterior end of the heart were fastened to its enveloping membrane. I 
am rather uncertain whether these fibres are united together by a mem- 
brane, but appearances are more in favour of the idea that it is not present 
in this species j at all events, the wall next the stomach is deficient, neither 
is there a cell present at either end ; Mr. Ray Lankester’s opinion, there- 
fore, that the heart is a mere churning organ is so far confirmed.” 
The Slit in Focus of an Object- ylass. — In the 11 Monthly Microscopical 
Journal” for Nov. Mr. Wenham, P.R.M.S., makes the following remarks. 
He says finally: “A few words concerning the slit in focus of object- 
glass, for cutting oft* all these disputed or false rays. It is difficult to 
annihilate this by theory. Having given the death-blow to Mr. Tolles’s 
extra apertures, it may be treated with but little notice, but cannot be got 
rid of as a thing of no account or a mere sensational affair.” Col. Woodward 
says : “ This method might be used without giving rise to material inac- 
curacy when the objective is adjusted for uncovered objects ; but when it is 
closed to the point of maximum aperture its spherical aberration is of course 
no longer corrected for uncovered objects.” In measuring varying angles of 
aperture by the usual method, we take them at all points of the adjusting 
collar, and do not place in front a thickness of glass suitable for that cor- 
rection, because with a parallel plate of glass there is no perceptible differ- 
ence. The angle at the crossing point of the rays is the same whether it is 
there or not. I stipulate that the edges of the stop shall be in the crossing 
point. If anyone thinks proper to introduce an intervening plate of glass, 
serving no purpose, he must focus through it, so as still to get the stop in 
the focal plane. Further, if the collar is set for an object immersed 
balsam, for the purpose of testing its reduced aperture therein by the means 
I have described, the slit must be set in focus, w r hether air, water, or balsam 
is the intermedium. In Mr. Tolles’s fth the immersed aperture was found 
to be the same with all three, simply because they are parallel plates. 
Mow to Mount Diatoms Quickly but Roughly. — Although most micro- 
