170 NOTES AND MEMORANDA. [^'o^mL.S^Tff 
of parallel rays can be thrown upon the object at any required degree 
of obliquity." He found the Podura beads were shown with it very 
satisfactorily, and he states that " it has the advantage of regulating 
obliquity to a great nicety, and giving a pencil of rays producing in 
some cases a fine definition." The illuminator itself consists of 
three plano-convex lenses, one of which, that on which the light falls, 
is of 2-inch focus, and the two others of which have respectively a 
1-inch and a ^^-inch focus. The large and the small lens are adjust- 
able by means of a sliding cap. The movements of the illuminator 
(which can be effected with any degree of obliquity) are provided for 
by a graduated arc, which fits beneath the stage of the microscope, and 
on which the illuminator works. 
A suggested Plan for Dark-ground and Oblique Illumination. — 
A correspondent gives the following brief description of a method 
employed for the above purpose : — Let a square be cut parallel to the 
base from a square or hexagonal pyramid of glass, grind and polish 
its edges to four or six different angles to suit various object-glasses. 
Drill a hole through its centre, and mount with slotted diaphragm 
to fit substage. The prism thus formed may be plain or tinted glass, 
any edge may be achromatized by a small flint prism or cylindrical 
lens of requisite curve cemented to its under-surface. The bull's-eye 
condenser, and stage-mirror are used for illumination. A flat line of 
light is thrown obliquely upon the object, well adapted to bring out 
markings on tests. Cylindrical lenses might also be employed for 
the same purpose. 
Prizes for Collections. — The Committee of the Belfast Naturalists' 
Field Club offer a number of prizes for the best collections of various 
Natural History objects, Some of the prizes are valuable. The one 
devoted to microscopy is not large, but it may eventually develop 
into something better. It is a prize of 10s. for the best set of twenty- 
five microscopical slides. Prizes to be competed for in March, 1870. 
Messrs. Cassell's Estimate of Microscopic Perfection. — Messrs. 
Cassell announce the fact that, in accordance with a want expressed 
by readers of their ' Popular Educator,' they " have arranged to supply 
through the ordinary trade channels, at exceptionally low prices, a 
series of first-class microscopes, many of them designed on new and 
improved principles, for school use and private study" ('Echo,' 
Aug. 11th). The following is a description of these first-class instru- 
ments (!) : — " The Society of Arts' Prize School Microscope, with rack 
adjustment, 3 powers, giving 656 superficies, with condenser, ani- 
malcule-cage, pliers, &c., 10s. Qd. Ditto, with body and eye piece, con- 
verting it into a compound microscope, 13s. Superior School Micro- 
scope, with joint, achromatic, magnifying powers 400 to 8000 times, 
in neat mahogany cabinet, 21s- Achromatic Microscope, all brass, 
3 powers, magnifying 50, 100, or 180 times, in mahogany cabinet, 
38s. Achromatic Microscope, on bronze stand, very powerful, 45s. 
Ditto, ditto, larger, 60s. Compound Achromatic Microscope, on 
bronze stand, to incline at any angle, with rack-work adjustment, 
stage, and diaphragms, 63s." Those who know anything of micro- 
