24:0 On the Preparation of Bock Sections S'TmS 
thus formed, of any desired diameter and depth, by selecting the 
size of cell accordingly ; and the wall being continued all round, 
gives the important advantage that the slide can be laid down flat, 
without any risk of water escaping ; the water is, indeed, held so 
securely in the covered half of the cell, that it may be turned upside 
down without the object being displaced. 
The water in the trough is readily renewed, and the living 
object refreshed at any time, by simply laying down the slide flat, 
and filling up the whole cell with water from a pipette, when the 
object floats out into the open part ; and the surplus water is again 
readily withdrawn by a pipette. 
The trough is originally filled, and the object placed in it in the 
same manner on the flat; thus allowing the entire area of the 
open half of the cell for this purpose, instead of only the narrow 
opening into the trough ; and this affords much facility in the case 
of delicate objects, such as cast larva skins, that will not bear 
handling, and have to be floated into their place. Then, to empty 
the trough, it is held obliquely over a watch-glass, and a small jet 
from a curved pipette washes out the contents of the trough safely 
into the watch-glass. 
The only practical objection found to these small troughs is, 
that they are too small to be cleaned inside ; but this difiiculty is 
completely got over by simply never letting them get dry, so as to 
prevent any film forming upon the glass, by keeping the troughs 
always immersed in a tumbler of clean water. 
These troughs have the important advantage that any one can 
readily make them ; the thin cover-glass is easily cut in half by 
scratching it across with a writing diamond whilst laid on a glass 
slide, wetted to make it stick close, and then shifting the cover- 
glass to the edge of the shde, with one of the halves projecting 
over, when it is separated by a light touch. 
XI. — On the Preparation of Roch Sections for Microscopic 
Examination. By David Forbes, F.K.8., &c. 
The results of microscopic investigation into the mineral character 
and physical structure of rocks have of late not only proved how 
essential an instrument the microscope is for inquiring into their 
origin, and studying the various changes which they have under- 
gone, or are now undergoing, but also point out that its future 
and more extended application must throw great light upon many 
abstruse questions relating to the geological phenomena of nature, 
by the careful study of minute microscopic details which until lately 
have been quite neglected or overlooked. 
