706 MICROSCOPY. 
preparation may be kept within it, for days or weeks together, 
without losing vitality, owing to the simple arrangement for sup- 
plying fresh air. 
We have repeatedly had the opportunity of witnessing the use 
of this slide, and are convinced that nothing of the kind has yet 
been devised which can equal it in excellence, either for observing 
or generating the lower forms of life—Journal Franklin Ins itute. 
To Bracken Brass. — The following methods are given by 
anonymous correspondents in the “ English Mechanic and World 
of Science.” Though not new they will be useful to readers, who 
desire to give a dead-black finish to adapters, diaphragms, ete. 
Warm the brass over a gas flame or spirit lamp, and plunge it 
while hot for two or three seconds into nitric acid. Then heat 
again until it blackens, brush off the blisters, and lacquer if a 
lustrous surface is desired. Instead of the nitric acid the follow- 
ing fluid may be used: a mixture of two parts of arsenious acid, 
four parts of hydrochloric acid, one part of sulphuric acid, and 
eighty parts of water. ; 
MONOCHROMATIC SUNLIGHT, BY MEANS OF GLASS Prartes.— Mr. 
J. Edwards Smith, of Ashtabula, Ohio, has obtained light with 
which he is perfectly satisfied by means of a light sky-blue od 
darker green glasses. He prefers to use one blue glass combin 
with two or three green ones, the best shades being ascertained 
by trial. Several such sets, of different depths of color, may ™ 
mounted in a series, like magic lantern pictures, 80 that either 
set can be brought easily over the hole in the shutter. By sunlight 
transmitted through such a combination of glasses, and without 
condenser or apparatus of any other kind, he “‘ resolves ” all the 
shells of the Probe Platte with perfect ease. He considers ers 
light thus modified as good as the more nearly monochromatle 
light of the troublesome ammonio-sulphate cell. 
An Orno Exprrment.— A correspondent of the © Scien- 
tific American ” thinks the photographic camera might be a P 
of gaining an unlimited magnifying power. He would pho h pay 
an object and then take a series of enlarged views, each OF. 
representing on an enlarged scale, a portion of the preceding an 
Evidently he is not accustomed to the use of magnifying pares 
Campnor 1N Pararrtn Lames.— Mr. Jobn A. Perry; of 
