52 
THE YOUNG SCIENTIST. 
grow after being detached. In one tank 
it lived for over four years. 
F. antipijretica (Fig. 4). Found in springs 
and lakes. Is of a light green color, and 
is very hard to kill. I have thrown it out 
of the aquarium into the garden, where it 
was exposed to the hot sun all summer, 
Fi^Jf. 4.— FONTINALIS ANTIPYEETICA. 
but when placed in the water it started 
forth into a bright green mass again. All 
the varieties of this order of plants I con- 
sider to be equal to any plants now in use, 
they will stand so much abuse, particu- 
larly all the small leaved varieties, that 
grow in lakes and standing water. They 
will stand a high or low temperature, and 
flourish both winter and summer. 
How to Grind Convex Lenses— II. 
BY E. M, BAILEY. 
TTAVING selected a piece of crown glass, 
cut from the plate a piece somewhat 
larger than the proposed lens, and with a 
pair of nippers trim it as nearly as possi- 
ble to a circular shape. The glass must 
be cemented to the end of the wooden 
centre. For this purpose use a cement 
with shellac as a basis, called by jewelers 
black cement. It is sold in the form of 
sticks. Melt the end of this in a lamp 
flame, and smear it on the wood, thinly 
but evenly. Pick up the glass and hold 
it in the lamp until it is quite hot, and 
then holding the cemented end of the 
wooden centre in the flame so as to re- 
melt the cement, place the hot glass on 
it ; press them firmly together, centering 
the glass on the wooden head as accur- 
ately as possible. When cold the glass 
is held so firmly by the cement that it 
will break more easily than it can be 
pulled off. 
Put the slide rest and tail stock of the 
lathe out of the way, take out the centre 
spur, and insert the wooden spindle in the 
mandrel, where it must fit tightly, so as not 
to work loose during the grinding process. 
Take a small emery wheel, or a piece of 
a broken wheel, in the right hand, set the 
lathe in motion, and hold the emery stone 
against the glass, grinding toward the 
edge at first, and w^orking it into a con- 
vex shape. During the grinding, the glass 
should be kept wet with water, which may 
be conveniently done by touching it fre- 
quently with a wet sponge held in the left 
hand. It must not be allowed to get so 
hot as to melt the cement. After it has 
been worked into a smooth convex sur- 
face, stop the lathe and apply the concave 
gauge to it, which will show where the 
grinding is to be continued in order to 
produce the right curvature. The gauge 
test should be made frequently after the 
glass has nearly reached the intended 
size. Toward the end, the work must be 
conducted very carefully, as two or three 
seconds' pressure of the stone upon the 
glass in one position will grind it out of 
shape. It must fit the gauge so that on 
removing the centre from the, lathe and 
holding it up before the eyes, no light can 
be seen passing between the glass and the 
gauge at any point. Keplace the centre, 
and with very fine emery paper go over it 
some time to remove the deep scratches 
left by the coarse stone, testing frequently 
with the gauge. Follow this with a piece 
of very soft whetstone. This will leave 
the surface in good condition for the pol- 
ishing process. Grinding a medium sized 
lens for a magnifying glass need not take 
over an hour, unless a needlessly thick 
piece of glass has been used. The hand 
soon acquires the correct way of moving 
