THE YOUNG SCIENTIST. 
71 
exact size of the lens. The ends of the tin 
strip should be made narrow and ham- 
mered into a half-round form, so as to clasp 
the end of the wire, which should then be 
placed in the tube thus formed, and soldered 
there. A little care will be required in mak- 
ing the exact measurements, and fitting the 
loop so as to get it just the right size to hold 
the lens. If it should be a very trifle too 
small, the lens may be ground down on a 
common grindstone. If it be too large, it 
may be filled up with a second ring of tin, 
or even cardboard. 
To fasten the lens in its place, various 
devices may be used. We fastened ours 
with sealing-wax, and it answers very well. 
This was done by heating the loop until it 
would melt the wax, and then coating the 
Fig. 3. 
inside of it with that material. While the 
wax was still soft, the lens was pressed to its 
place, and the loop was again gently heated, 
so as to cause the wax to adhere to it firmly. 
A more mechanical and artistic method 
would be to get two rings of sheet tin, each 
an eighth of an inch wide, and of just such 
a size as would fit into the inside of the 
loop. These two rings are to be soldered 
to the loop, the lens being between them. 
Various other ways might be suggested, but 
the sealing-wax answers very well. 
The next step is to connect the lens and 
its frame with the stand, and this is done 
by means of the cork. Two holes are bored 
through the cork, as shown in Fig. 2, one 
passes lengthwise through the centre, and 
the other hole passes through the cork at 
right angles to the first, and a little to one 
side of the centre. These holes can be 
made by means of a cork-borer, which is a 
small tube of sheet tin or brass, with the 
edges sharp; or they can be made by first 
passing an awl through the cork, and then 
enlarging the hole to the proper size by 
passing a red hot wire through it — an old 
knitting needle answering very well. As 
soon as the holes have been bored, the 
whole may be put together as shown in 
Fig. 3, and the condensing lens is com- 
plete. 
The simplest and most obvious use of 
the condensing lens is to throw light (either 
daylight or lamplight) on opaque objects, 
examined by reflected light; and upon this 
and other uses we shall have a good deal to 
say in future numbers. 
A Fifteen-Cent Battery. 
Ed. Young Scientist— JjSiSt year I constructed 
a cell of the carbon battery very cheaply. All 
that is needed is a glass fruit jar holding one 
quart, a piece of carbon (charcoal), and a piece 
of zinc. An old can will answer, or take a 
large quart bottle, and, with a glass cutter, cut 
the top off, which will make a good jar. You 
can get an old zinc at almost any telegraph 
station, or you can roll one out of sheet zinc, 
which will do as well. Make it 1 inch in di- 
ameter by 6 inches (or the height of can). Go 
to a tin shop and they will give you a piece of 
carbon (charcoal) same size as zinc; solder a 
good wire to the zinc, and drill a hole in the 
carbon and place a wire in it; place between 
the carbon and zinc a couple of pieces of rub- 
ber or wood; place a tight rubber band around 
them to hold them, and place them in the jar. 
The solution is made by heating one quart 
of rain water to the boiling point, adding three 
ounces of bichromate of potash, and stirring 
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