THE YOUNG SCIENTIST. 
59 
see a better way. But the great demand 
that has arisen for binocular prisms has 
induced the makers to discover a plan of 
working them in blocks, a number at a 
time, the particulars of which I do not 
pretend to explain. 
Some very excellent prism work is pro- 
duced on the Continent, and as the mode 
of polishing is peculiar, it may be worth 
while to record it. Chevalier & Co., of 
Paris, through Messrs. Beck, politely sent 
me an explanation of the process, together 
with a sample of all the grinding and 
polishing materials used in their busi- 
ness. After the surface of the prism is 
smoothed, a piece of very thin, smooth 
paper (much resembling photographic 
negative-paper) is cemented by its extreme 
ends with a little gum or dextrine to the 
metal lap ; a lump of yellow tripoli (la- 
belled " Tripoli de Yenise ") is then 
rubbed dry over the paper, and the 
prism, also dry, polished thereon by hand 
movement ; generally not more than two 
or three applications of the powder are 
required. I have tried this method with 
the identical paper and polishing ma- 
terial, but must state that, in my hands, 
the result has not been satisfactory for 
accuracy, at least in very small prisms ; 
for larger ones it may answer better. 
Management and Selection of Aquaria. 
BY A. W. EGBERTS. 
I CANNOT urge too strongly the wis- 
dom of beginning the study of aquaria 
on a small scale at first, so that the funda- 
mental principles may be mastered before 
much money has been expended on a 
large and expensive tank, which will, in 
all probability, result in discouragement 
and failure. 
For the first lesson I would advise the 
use of a candy jar (see engraving). Hav- 
ing filled it with soft water, expose it to 
the full action of sunlight from two to 
four days, at the expiration of which time 
the spores of various kinds of confervas 
contained in the water will have been de- 
veloped, and the inside of the glass jar 
will have become thickly coated with con- 
fervae. Continue to expose the jar to the 
.full force of the sun's light and heat 
ray's, no matter how high a temperature 
the water may attain, and a second de- 
velopment of plant life will take place, 
which will be known by the water becom- 
ing of a thick opaque green color, from 
the presence of millions of niicroscopi ■ 
plants, which have the power of movin.s 
about in the water. These facts, and man 3; 
TALL JAE FOR AQUARIUM. 
others which might be cited, go to prove 
how universal is the diffusion of the 
spores or seeds of low forms of animal 
and plant life, which only require the 
proper conditions of light, heat and air to 
develop them. 
We will now take this jar and place it 
where it will receive cool diffused daylight, 
but no direct sun rays, and will also place 
a dozen snails in the jar, a fresh- water 
mussel, and a small bunch of mermaid- 
weed. In three days the water will have 
become perfectly clear. In consequence 
of the lowering of the temperature of the 
w^ater in the jar, caused by depriving it of 
the direct action of sunlight, the green 
coloring substance, or locomotive plants, 
contained in the water, have ceased to 
exist as living plants, but their remains 
may be found deposited on the bottom of 
of the jar in the form of a dirty-looking 
loose brown substance. This deposit of 
decayed vegetable matter is termed 
" humus," and is quite harmless in an 
aquarium. The fresh-water mussel has 
also helped to destroy immense quantities 
of these vegetable organisms, and if you 
watch him with a magnifying glass you 
will see a constant stream of water loaded 
