498 MICROSCOPY. 
Tin cells. Mr. Richards had used cells of rather thin tin foil, ; 
cut out with two punches with a piece of tube between to keep 
them the right distance apart: these cells were fastened on bya : 
solution of glue and treacle dried on and. then moistened enough i 
to stick them, the cells being so thin that any liquid cement would " 
have run in. The chairman commended the tin cells introduced 
by Mr. Suffolk; he having used them, fastening them on with 
marine glue with great satisfaction: Dr. Matthews, however, 
objected to them because they melt so easily if the slide be over- 
heated. 7 ; 
Zinc cells and vulcanite cells were favorably mentioned by Mr. , 
White, the former bearing great heat without melting, and the 
latter resisting the action of acids: but Mr. McIntire found they : 
had a tendency to chip off. $ 
[The expensiveness of glass cells, when used in large quantities, 
is the continual occasion of a demand for some good substitute. ; 
Tin cells are largely used in this country, being often fastened on 4 
by gold size whose only fault is that it dries so slowly that the 
cells require to be fastened on long before using, or with dammar 
varnish or Bell’s cement. Doubtless the lead cells will hereafter 
be used by many who desire to preserve a great many specimens : 
but cannot afford to spend unnecessaril y on an elegant mounting. 
It would seem that some of the dealers might prepare and sell 
them at a price that would be remunerative to themselves, and 
at the same time an accommodation to buyers. | 
Tue Common PARABOLOID as AN Immersion INSTRUMEN 
Notwithstanding the introduction into use of special on 
as immersion paraboloids, it may not have occurred to all wn ú 
the microscope that the ordinary form of parabolic ilumin 
capable of being used wet with excellent results. Placing a 
microscope in a vertical position, and greasing the rod in the the 
tre of the paraboloid to keep the water from running out pe 
side of it, the cup of the paraboloid is filled with water heap anti 
as far as can be without running over, and then brought up" 
the water comes in contact with the under surface of piae” 
The direction of the rays leaving the paraboloid is not We pore 
this arrangement, but dispersion at two surfaces is av? and at 
the rays enter the object slide without the usual refrachon™ y 
such an angle as to suffer total internal reflection before 7 © — 
