MICROSCOPY. 509 
aperture would correspond to the focus of the lens, and the nozzle 
would just include the cone of rays capable of forming an image 
and would exclude all false rays of any considerable angle. This 
method would be inconvenient, however, and as the angle is meas- 
ured by a horizontal movement a vertical slit will be a satisfactory 
substitute. For high powers the slit must have thin edges; and 
it must be capable of adjustment to the width of focus of the lens, 
His arrangement is easily made and used. A plate three inches 
long and one inch wide has a central aperture nearly one-half inch 
wide, the edges of this opening being bevelled away below so as to 
admit a large angle of light. Upon this plate lies a glass slip 
about 2 in.X4 in., pressed against at one end by a spring, and at 
the other end by a screw, so that it can be easily slid backwards 
and forwards under the two staples (one inch apart) which hold it. 
upon the surface of the plate. The slip is formed by the edges of 
two slips of platinum foil (+001 thick) one of which is cemented 
with Canada balsam upon the glass slip, while the other is fastened 
under one of the staples so as to lie on the glass slip but not move 
with it. These platinum slips never overlap ; but their edges may 
be brought in contact, or may be separated as widely as desired by 
means of the set-screw pressing against one end of the glass slip 
which carries one of them. In measuring angles the usual method 
of rotating the instrument horizontally is employed; only this 
apparatus lies upon the stage with its slit in focus of the objec- 
tive and adjusted in width so as barely to include the whole 
breadth of the focus. If the stage of the microscope is too thick 
to admit full angle of light, the apparatus may be arranged below 
the stage and the objective focussed down to it. 
CataLoguixe Microscopic Specimens. — At the Medical Micro- 
Seopical Society, a paper by Mr. Groves was read on the subject 
of cataloguing and arranging microscopic specimens. Though 
Classification was deemed necessary in large cabinets, yet it was 
Considered entirely undesirable in small ones, and in both cases 
the catalogue and not the arrangement was relied upon for finding 
objects. The method of cataloguing recommended consists of an 
ordinary alphabeted note book in which, under the proper alpha- 
l heading, every portion of each specimen is independently 
entered and the slide referred. to by numbers or otherwise. Thus 
one excellent slide which shows well a number of points of struct- 
