CORRESPONDENCE. 311 
perspective, and rarely with both faces of the shell, they too often fail 
to satisfy the student, though the artistic labour bestowed on them has 
been exact and conscientious. 
( To be continued.) 
I remain, Sir, yours, &c., 
T. Rupgrr Jones. 
Mr. Arcuer’s Genus HyaLospHENta. 
To the Editor of the ‘ Monthly Microscopical Journal, 
READING, May 5, 1877. 
Sir,—Mr. Archer, in his “ Résumé of Recent Contributions to our 
Knowledge of Fresh-water Rhizopoda” in the current number of the 
‘ Quarterly Microscopical Journal,’ refers “a lobose, monothalamian 
sarcodine” to the genus Hyalosphenia, instituted, with questionable 
propriety, by Stein, and accepted by Mr. Archer, though with obvious 
hesitation. 
Permit me to say that the so-called Hyalosphenia lata recently 
detected by E. Schultze is neither good as a genus nor new as a species, 
having been briefly described and correctly figured in your own Journal 
so far back as December 1870, under the name of Difflugia ligata by 
Yours truly, 
J. G. Tate. 
Lorp 8. G. Ossorne’s Exursrror. 
To the Editor of the ‘ Monthly Microscopical Journal.’ 
SIDMOUTH. 
Sir,—As I find that my former letter to you on the subject of 
an instrument to facilitate the exhibition of the Diatomacex, and other 
objects, requiring very oblique light when examined with high powers, 
has attracted the attention of some good observers, will you kindly 
permit me to offer a few more words on the subject, as I am anxious 
to prevent any disappointment to those who may try it. 
I find, and have informed Mr. Curteis, that all the apertures 
necessary, may be with ease cut on one disk. If a line is drawn 
through the centre of a disk, crossed by another at right angles, a 
good workman will with ease cut four apertures, one on each of these 
lines. They should be one-tenth of an inch from the point at which 
the lines intersect. If so cut, when the disk is revolved, each aperture 
will in turn come to the same position as regards the slide on the 
stage; they will not in any way interfere with each other. This 
plan saves a great deal of trouble, and enables the observer to try the 
effect of each aperture on any object, with the greatest ease, and no 
disturbance of any part of the instrument. 
I always now use four apertures so cut—a fine slit, and three 
triangles punched in different positions as regards the centre of the 
disk; in the case of each, taking care that their illuminated edge 
should be the one-tenth of an inch from the centre. 
It much facilitates the use of the Exhibitor, if, before this disk is 
