92 THE NORTHERN MICROSCOPIST. 

Jastiola, speciosum, &c., and Spirulina Jenneri—while the Animal 
Kingdom is represented by :— 
Trachelomonas bulla, 
Ophiocoma neglecta. 
Lelotrochidium crateriforme. 
Tubifex rivulorum., 

Ameeba. Hloscularia cornuta. 
Acineta grandis. Polyphemus pediculus. 
Sertularia pumila, Canthocamptus minutus. 
Aglaophenia pluma. Dorts tuberculata. 

Lolis Landsbursgitz. 
There is a marked improvement in the general style of the 
drawings. Has any reader taken them as the basis of a good 
drawing in color ? 

ON A METHOD OF MOUNTING THE LIMITING 
APERTURES FOR INCREASING THE PENE- 
TRATING POWER OF OBJECTIVES. 
To the Editor of the “ Northern Microscopist.” 
S1r.—Since sending you the two forms of apparatus for limiting 
the angular aperture of objectives (figured in this number) I have 
employed a double nose piece as a receptacle for the circular 
limiting apertures when in use. A shallow recess or rebate is 
made in the top edge of one of the female screw parts of the nose 
piece, thin metallic discs with holes of suitable diameters are made 
to fit the shallow recess, and where the nose piece is attached to 
the microscope and the objective is screwed into its place a disc 
with the required aperture can be dropped into the recess by merely 
moving the arm which carries the objective on one side. They 
can also be easily lifted out of the recess by means of a wire hook, 
thus obviating the necessity of touching them with the fingers. 
Each disc is numbered, so that when a satisfactory limiting aperture 
has been selected by experiment the number on the disc may be 
noted for future use. The metallic discs are packed in a small 
brass box along with the wire hook. The plan now proposed has 
several advantages over the sliding shutter apparatus, which I 
used for the same purpose some ten years ago, and will render the 
Iris diaphragm and the forms figured in this number unnecessary to 
those microscopists who possess a double nose piece. I have 
devised another simple form of apparatus for carrying circular 
apertures, but will not trespass more on your columns at the 
present time. 
J. B. DANCER, MANCHESTER, 


