NOTES AND CORRESPONDENCE. 
Pleurosigma angulatum.—It is a difficult question to decide, 
and one which has not yet been determined with complete 
certainty, as to whether the small spaces or areole which the 
striee of Pleurosigma angulatum present are concave and their 
contour in relief, or, on the other hand, whether the edges 
form furrows, and their spaces projections. I know that 
distinguished observers have maintained the two propositions, 
but, as far as I am concerned, I believe that there exist 
neither depressions nor elevations on the valves of P. angu- 
latum ; it is uniformly an optical illusion, produced by the 
shadow which the strie cast when viewed in a certain way, 
and which, in my opinion, are not straight lines, but lines 
slightly broken. The appearance of a hollow or of a relief 
is particularly due to this circumstance, that the strie are 
discontinuous ; and what tends to confirm the opinion which 
I offer, that on the valves of this diatom neither hollows nor 
projections really occur, is, that according as the focus is 
changed, the areole appear at one time dark with clear out- 
lines, at another time clear with dark outlines. This is 
exactly the effect which is produced if a micrometer is ex- 
amined and the focus of the lens yvaried.—Movcuest, Roche- 
fort-sur-mer. 
Collins’ Mounting and Collecting-cases—Mr. Collins, of Great 
Tichfield Street, has introduced a very complete mounting- 
case, which must prove useful to microscopists, especially so 
to those who devote a great deal of attention to the prepara- 
tion of specimens. It contains a Shadbolt’s turn-table, brass 
table, spirit-lamp, pipettes, spring clips, wooden clips, 
_ tweezers, tin cells, balsam, marine glue, asphalte, turps, gold 
size, thin glass covers, glass slips, and five extra bottles. 
Another box, more particularly adapted for anatomical pur- 
poses, includes a neat injecting apparatus. 
Mounting-cases are too often of an expensive character. 
We therefore call particular attention to Mr. Collins’, as it 
is compact in all its arrangements, and sold at a moderate 
price. It is by such aids as these that the working micro- 
scopist is enabled to pursue his investigations with saving of 
time and increased satisfaction to himself. 
