BY FRANK CHASEMORE.
The investigation of astronomical phenomena can only be made with the
aid of a good telescope, the purchase of which is attended with
considerable cost. It is my purpose in this chapter to give such
directions as will enable any boy with average ingenuity to make for
himself, at the cost of a few shillings, an instrument with which he can
observe the more interesting of these phenomena.
This telescope will be of the simple non-achromatic class--that is, the
colour effect of the unequal refraction of light is not corrected.
Object-glasses of the achromatic construction are very expensive. All
refracting telescopes were of the simple class up to 1758, when Mr. John
Dollond, who had a few years before set up in business in London as
optician, discovered a way to correct the colour effect on the image.
This was by making the object-glass compound, or of two or more lenses
fitted to each other, each being made of a different quality of glass
from the other and having a different refracting power, one lens
correcting or neutralising the dispersing caused by the other. The
lenses for our telescope can be had from Messrs. Dollond and Co.,
descendants of the above John Dollond, No. 1, Ludgate Hill, who have
given me some valuable hints with regard to the construction of this
telescope.
A refracting telescope consists of an eye-piece, a tube, and an
object-glass; these are mounted on a firm stand. The object-glass at one
end of the tube collects the rays of light, reflected from an object, to
a point, in the focus of the eyepiece, which magnifies the image that is
there formed, enabling the eye, placed at the orifice of the eyepiece,
to see an enlarged image of the object.
[Illustration: Fig. 1.]
The stand must be firm, so as not to vibrate when any one passes along
the floor of the room, and it must have a vertical and lateral motion
connected with it. Fig. 1 shows what our instrument will be like when
finished.
The first thing to be made is the tube; this must be thirty-nine inches
long and two inches in diameter inside. Get a wooden roller four feet
long and two inches wide. A piece of curtain rod will do. Now mix some
strong glue. If you have not a glue-pot, mix it in a jar placed in a
saucepan of water. Get some sheets of stout brown paper and well damp
them. Take a strip of brown paper, that has not been damped, thirty-nine
inches long and seven inches wide. Rub the roller all over well with
powdered chalk and put this dry paper strip round it to form a case,
lapping and gluing the edges together, but being very careful not to let
any glue touch the roller. Now take your damped paper and rub it all
over on both sides with hot glue, and roll it on the roller; roll it
tightly and rub the glue well in, and rub each layer of paper well in to
the under one, so that when dry it will form a mass of paper and glue.
Put on enough paper to form a casing a quarter of an inch thick. When
you have done papering set the whole on one side to get quite dry and
hard. While this is drying we can be making the eyepiece and stand. For
the eyepiece we shall want a piece of brass tube four inches long and
large enough for the larger of the two lenses that form the eyepiece to
go inside; that will be a little more than an inch in diameter inside.
Get your lenses before getting the tube. This tube can be bought at the
ironmonger’s.
Now for the lenses. Go to Messrs. Dollond and ask for a two-inch simple
object-glass, forty inches focus. This will be one shilling and
sixpence. For the eyepiece ask for two plano-convex lenses--one of
one-inch focus, the other of two-inch focus. These will be three
shillings and sixpence the pair. The object glass is to be double
convex. Now, having got your lenses, we will fix them in the tube.
[Illustration: Fig. 2.]
Cut a piece of cardboard three-quarters of an inch wide, and long enough
to go all round inside the tube tightly, and not to lap. Push this in to
form a lining at one end, and forming a shelf out of the thickness. This
shelf is to be about two inches from the end. Now turn the other end of
the tube up and drop the larger of the two lenses--which is called the
field lens--on to the shelf with the rounded side downwards. Now push in
on the top of it another cardboard lining three-quarters of an inch
wide. Push this lining quite down on to the flat side of the lens to
keep it firm. On to the shelf formed by this lining place a disc of
cardboard the size of the inside of the tube, and with a hole cut in the
centre half an inch in diameter. This hole must be cut quite clean. On
to this disc push in a cardboard lining one inch wide to keep all firm.
Now cut two discs of cardboard, one the exact size of the inside of the
tube, with a hole in the centre a trifle smaller in diameter than the
small lens, which is called the eye-lens; the other a quarter of an inch
smaller, and having a hole in the centre the exact size of the eye-lens.
Glue these two discs together (as in Fig. 2), being careful to get them
concentric. When this is dry push the eye-lens into the ledge formed,
the flat side downwards, and put the cardboard discs on to the lining in
the tube, the rounded side of the glass inside the tube. Fasten the disc
and lens in place with a narrow strip of cardboard, going all round just
inside the tube.
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
Fig. 3 will show the arrangement of lenses and disc in the tube. The
lenses are to be an inch and three-quarters from edge to edge; the disc
is to be an inch from the eye-lens. Now get a tinman to make you a cap
of thin brass plate (like Fig. 4). This is to fit tightly on the end of
the tube over the eye-lens, and is to have a hole in the centre of the
top three-eighths of an inch in diameter.
Now the eyepiece is finished, and we will get on with the stand.
[Illustration: Fig. 5.]
Let two circular pieces of beech five inches in diameter and one and a
half inches thick, each to have a hole bored in the centre an inch in
diameter, and going right through the wood. Take one of these pieces,
and on the under side fasten, by three strong brass hinges that work
stiffly, three legs, made of inch and a half square pine (Fig. 5). These
legs are to be four feet six inches long.
[Illustration: Fig. 6.]
Take the other piece of beech and fasten a rod or roller of wood, two
feet and a half long and one inch diameter, into the hole, so that one
end is flush with the top side of the wood (Fig. 6). Now fasten two
uprights to the top, letting them into the wood (Fig. 6). These are to
be three-quarters of an inch thick, and four and a half inches high, and
are to be three and a half inches apart.
[Illustration: Fig. 7.]
Bore a hole in each upright about half an inch from the top and about
one quarter of an inch in diameter. Now get nine screw stair-eyes at
the ironmonger’s, and about seven feet of brass wire one eighth of an
inch thick. Get this wire straightened. Into the inside of each leg, and
one foot from the top, screw one of the brass stair-screws. Now get a
piece of wood, circular, three inches diameter and one and a half inches
thick (a ribbon roller will do very well), and bore a hole right through
the centre, one and a quarter inches in diameter. Round this piece of
wood at equal distances screw three more of the screw-eyes. Now cut from
your brass wire three lengths of fourteen inches, and turn a ring at
each end of each piece, and hook one end of each piece into a screw-eye
in the circular block, and the other end into the eye in each leg,
closing up the rings so that they will not come unhooked. This
arrangement will keep the top from tilting. Now the stand is finished,
and we will take the tube in hand if it is quite dry and hard. Before
drawing out the roller, cut the ends off quite square with a sharp
knife, leaving the tube thirty-seven and a half inches long. Now draw
out the roller without breaking the tube. We must next fix the
object-glass. Cut a strip of cardboard half an inch wide, and long
enough to go all round inside the tube without lapping, and to fit
tightly. Push this inside, so that it will form a shelf half an inch
inside the end of the tube. Glue this in its place. Upon this place the
object-glass, and fix it there by gluing a strip of cardboard all round
inside the tube on the top of the lens. To fix the eyepiece, cut from
the roller used to make the tube on, a piece one inch long, and bore a
hole right through the centre of it the exact size of the eyepiece tube.
Glue this block in the other end of tube. Push the brass tube in this
hole with the field-lens inwards. The telescope can be focussed by
pushing in or drawing out the brass tube. Get a piece of deal eight
inches long and three and a half inches wide and two inches deep. Cut a
groove along the top as Fig. 7, one inch deep and a little more than two
and a quarter inches wide, to fit the outside of the tube. Glue this
block on the tube, so that one end is thirteen inches from the front end
of the tube (the eyepiece end). Put the block in its place between the
uprights on the stand, and fix it there by two screws passing through
the holes in the uprights and screwing into the block. Pass the rod
attached to the uprights through the hole in the top of stand and
through the hole in the block underneath.
[Illustration: Fig. 8.]
Now we have only to make the arrangement for elevating the telescope.
For this you will want the rest of the brass wire and the remaining
three stair-eyes as well as two pieces of thin brass plate, four and a
half inches long, half an inch wide, and one-sixteenth of an inch thick.
Bend these pieces of plate as in Fig. 8, making the bent parts one inch
long, and get the ends cut as in the figure, and have holes drilled in
the bent parts a little larger than the brass wire. Cut your wire into
two lengths of eighteen and a half inches, and take them to the tinman
and get him to cut a screw-thread nearly the whole length of each,
leaving about two inches to each. At this end of each get him to turn a
ring, and get him to close these two rings into one of the screw-eyes.
Get him to make a screw-nut for each wire about the size of a farthing,
but about twice the thickness. Screw the eye carrying these wires
through the tube into the eyepiece block, screw the remaining two
screw-eyes into two of the legs of the stand, on the outside of each leg
and about one foot from the top of each. Bend the cut part of the brass
plates into rings and close them in these screw-eyes. Now put the
screw-nuts in their places in the brass plates, and put the screw wires
through the holes in the top, and turn the nuts to the left, which will
draw down the wires and with them the eyepiece of the telescope. To turn
the telescope to the left turn the right-hand nut to the right, and the
other to the left, and to turn it to the right reverse the action of the
nuts. In making this telescope you must be very careful in fixing the
lenses. They are to be placed so that the centres are to be in one
straight line, which line is to be at right angles to the lenses. You
can cover your tube with coloured paper to give it a finish. It will be
advantageous, in using the instrument, by keeping out all light not
wanted, to make a cardboard tube about six inches long and large enough
to slide easily on the end over the object-glass and to project about
five inches. The telescope is now finished, and will with ordinary care
last for years.
[Illustration: Fig. 9.
Appearance of the Moon as seen through one of these telescopes on June
9th]
It may be well to add that astronomical telescopes show the image
inverted; this is done to save the use of erecting glasses which absorb
light, and consequently make the image faint. If you wish to make yours
useful for terrestrial purposes, you must insert in front of the field
lens, and about four inches from it, a lens of the same focal length;
this will turn the image right way up. The blurring of the image is
sometimes caused by the two lenses composing the eye-piece _not_ being
at the distance apart proper for their focal lengths.