THE YOUNG SCIENTIST. 
79 
heated to the boiling point, and kept at 
this temperature for some time— say half 
an hour to an hour— care being taken not 
to allow it to burn. The articles are 
allowed to cool in the milk, and when 
taken out are wiped dry and allowed to 
stand for a day or two until the cement 
has become quite hard. They may then 
be washed off with warm water, and the 
parts will be found to be strongly ce- 
mented together. The same milk may be 
used again, but not with such good effect. 
Generally, however, it is possible to pack 
quite a number of articles in the pan in 
the* first place, especially if they can be 
nested, or placed one within the other. 
Lens Making. 
AWEITEE in the ^English Mechanic thus 
describes his method of grinding and 
polishing lenses : 
So far as microscopic glasses are con- 
cerned, the modes usually'' proposed are 
very tedious, troublesome, and most un- 
certain. I have devoted many years to 
making for my own use achromatic and 
other lenses for microscopic object- 
glasses ; and as it may be useful to some 
who grind their own, I send a general de- 
scription of the way I go to work, first 
stating that I take from ten to twenty 
minutes to make a plano-convex from the 
rough, if no accident happens. 
I take a piece of glass and cement it to 
a chuck, and then turn it with a hard 
steel tool to the required radius. There 
is no trouble in doing this if the tool is 
wetted with saliva. I find this far su- 
perior to anything else. I then grind it 
smooth with a brass tool, and, after wash- 
ing, go on grinding it with the same emery, 
only covering the tool with a bit of silk. 
I use oil just at first, and then water. 
This will go some way towards polishing. 
I then clear away the emery, and work 
the glass on the silk with i^utty powder, 
now making the lens revolve for a little 
time till all scratches are nearly removed, 
and then finish it with the tool revolving. 
The reason for making the lens revolve 
is that the putty powder acts very much 
quicker, and so saves time and trouble. 
The emery I use is the common flour- 
emery washed once. I use the finest part, 
and this is quite enough, as I turn the 
lens to the right shape, so that it requires 
very little coarse grinding. While the 
lathe gives a circular motion, the hand 
should move backward and forward, 
changing the position of the lens. This 
gives a true figure. I use different silks 
for slight alterations in the focus. 
Something Nice. 
WE have just secured a few miniature 
"charm" opera-glasses, which we 
feel sure will please those of our readers 
that get one. These little opera-glasses 
are of white ivory, being single tubes 
about half an .inch long, and having an 
opening through them of about a six- 
teenth of an inch in diameter. On look- 
ing through this hole in the same way 
that we would look through a common 
spy-glass, there will be seen a portrait, 
view, or enlarged picture, of the Creed, 
Lord's Prayer, etc. These pictures are so 
small as to be quite invisible to the naked 
eye, but there is a powerful magnifier in 
the little ivory tube, and when we look at 
the pictures through it they appear to be 
quite large and very distinct. 
"We will send one of these postpaid to 
any of our subscribers that will procure 
for us another subscription, and send on 
the fifty cents. For all sums under one 
dollar we accept postage stamps at their 
full value. 
A Wise Woodpecker. 
Ed. Young Scientist— -Liast summer, while 
walking- in the woods, I found the nest of a 
golden-wing-ed woodpecker (Colaptes Auratus) 
built in the hollow of a dead tree. Wishing the 
eggs for my collection, I got a boy to climb the 
tree and collect the eggs for me, which he did 
with little trouble. A few days later, while 
walking by the same tree, I heard the wood- 
pecker hard at work deepening the hole which 
had previously contained the eggs, with a view, 
I suppose, of protecting its eggs in future from 
all mankind, which I have no doubt it succeeded 
perfectly in doing. 
Alexandee G. Gibbs. 
