18 
THE YOUNG SCIENTIST. 
smooth after being sawed. Most of the 
woods can be bought ready for the saw ; if 
they are not smooth enough give them a 
good sandpapering with the finest grade 
of sandpaper. All kinds of wood will 
warp or curl more or less if the conditions 
are favorable, but when this happens the 
warp or curl can be taken out by damping 
the floor or a board of suflicient size, and 
laying the hollow side of the warped stuff 
on it, and in a short time the warp will 
disappear. Another method is to dampen 
the hollow side with steam, then hold the 
opposite side to the fire until straight. 
The best woods for the beginner to use are 
pine, Spanish and red cedar, butternut, 
poplar, white-wood, bass-wood, and black 
walnut. The advanced amateur can use 
to advantage, white holly, mahogany, 
amaranth, ebony, tulip, rosewood, maple, 
cocobola, and satin wood. Besides the 
woods mentioned above, there are many 
other substances employed in fret-sawing, 
such as hard rubber, tortoise-shell, ivory, 
pearl, brass, silver, and gold. The prices 
of the woods mentioned above, in New 
York, range as follows per square foot, 
when one-quarter of an inch in thickness : 
Pine, 5cts. ; white-wood, 5cts. ; Spanish 
cedar, 12cts. ; bass-wood, 6cts. ; poplar, 
5cts. ; black walnut, Sets. ; white holly, 
16cts ; mahogany, 14cts. ; amaranth, 25cts. ; 
ebony, 50cts. ; tulip, 40cts. ; rosewood, 
20cts. ; maple, 14cts. ; cocobola, 25cts. ; 
satin wood, 35cts. 
There are also other woods than these 
that are made use of by the fret-sawyer, 
but they are scarce and costly, and I think 
the above-named will furnish a sufficient 
variety to fill the wants of most amateurs. 
Home-Made Telescopes and Micro- 
scopes— II. 
FLINT and crown glass for optical pur- 
poses are articles of commerce, and 
are sold by the dealers in optical goods in 
discs of all sizes and thicknesses. Ordinary 
sizes are sold by the pound, and as there 
is a tolerable uniformity in the qualities of 
the two kinds, as made by any one house, 
the young optician may safely make his 
curves to conform to those published, sat- 
isfied that he will get very nearly the de- 
sired results. It may be interesting to our 
readers to know that the optical glass of 
most of the great houses of Europe may 
be had in New York. Messrs. Chance & 
Co. have an agency in Duane St., and Mr. 
Emmerich, of Maiden Lane, keeps on 
hand a stock of French and German glass. 
In their first trials, however, our readers 
will probably use such kinds of glass as 
they can find, for although it does not re- 
quire a long apprenticeship to enable the 
young workman to turn out fair lenses, he 
must not expect to be able to do it without 
seme pratice. Therefore, it will be well to 
begin on simple magnifiers of medium 
size and power, and gradually pass to 
more complicated arrangements requir- 
ing greater skill and experience. The 
making of such lenses is by no means a 
waste of time, for besides the skill which 
is obtained, the lenses themselves are 
always useful for magnifiers, and when 
mounted in simple handles, a§ hereafter 
described, they are useful for dissecting 
microscopes, and make nice presents to 
such of our friends as have a taste for 
botany, entomology, etc. 
Such magnifiers may be very well made 
out of such pieces of common glass as are 
procurable almost anywhere. It is well, 
therefore, for the young optician to inform 
himself in regard to the different kinds of 
glass in ordinary use. 
All glass used for optical purposes must 
be free from striae and air bubbles. It is 
very evident that an air bubble would 
unfit a piece of glass for making a lens or 
a prism, and a streak of lighter or heavier 
glass running through a piece is equally 
bad, as it produces the same effect that is 
seen when we look through the heated 
air rising from a hot stove— objects seen 
through it are distorted. Glass which is 
free from striae or streaks is said to be hom- 
ogeneous, and it is this quality which it 
is so very difficult to secure in large 
masses, and which consequently limits 
the size of the object glasses of telescopes. 
In examining a piece of glass for striae 
we must be careful not to condemn it on 
account of wavy irregularities on the sur- 
face. These are all removed in working 
the glass, and do no harm. 
The streaks and bubbles which are 
