purchased for four or five shillings. But it may be easily made by the
young optician. Procure an oblong box, about two feet long, twelve inches
wide, and eight high. In one end of this a tube must be fitted containing
a lens, and be made to slide backwards and forwards so as to suit the
focus. Within the box should be a plane mirror, reclining backwards from
the tube at an angle of forty-five degrees. At the top of the box is a
square of unpolished glass, upon which from beneath the picture will be
thrown, and may be seen by raising the lid A. To use the camera, place the
tube with the lens on it opposite to the object, and having adjusted the
focus, the image will be thrown upon the ground-glass as above stated,
where it may be easily copied by a pencil or in colors.
[Illustration]
The form of a camera obscura used in a public exhibition is as follows:
D D is a large wooden box stained black in the inside, and capable of
containing from one to eight persons. A B is a sliding piece, having a
sloping mirror C, and a double convex lens F, which may, with the mirror
C, be slid up or down so as to accommodate the lens to near and distant
objects. When the rays proceeding from an object without fall upon the
mirror, they are reflected upon the lens F, and brought to fall on the
bottom of the box, or upon a table placed horizontally to receive them,
which may be seen by the spectator whose eye is at E.
THE MAGIC LANTERN.
[Illustration]
This is one of the most pleasing of all optical instruments, and it is
used to produce enlarged pictures of objects, which being painted on a
glass in various colors are thrown upon a screen or white sheet placed
against the wall of a large room. It consists of a sort of tin box, within
which is a lamp, the light of which (strongly reflected by the reflector
T,) passes through a great plano-convex lens E fixed in the front. This
strongly illuminates the objects which are painted on the slides or slips
of glass, and placed before the lens in an inverted position, and the
rays passing through them and the lens F, fall on a sheet, or other white
surface, placed to receive the image. The glasses on which the figures are
drawn are inverted, in order that the images of them may be erect.
THE CAMERA LUCIDA.
[Illustration]
This instrument consists of a glass prism, C, D, D, E, having four sides
covered. The sides C, D, being exposed to the object to be delineated,
rays pass through the glass and fall on the sloping side D, E; from
this they are reflected to the top, and finally pass out of the prism
to the eye;[6] now from the direction at which the rays enter the eye,
it receives them as if coming from an image at A, B, and if a sheet of
paper be placed below the instrument, a perfect delineation of the object
may be traced with a pencil. This is a very useful instrument to young
draughtsmen.
PAINTING THE SLIDES.
The slides containing the objects usually shown in a magic lantern, are
to be bought of opticians with the lantern, and can be procured cheaper
and better in this way than by any attempt at manufacturing them. Should,
however, the young optician wish to make a few slides of objects of
particular interest to himself, he may proceed as follows:
Draw first on paper the figures you wish to paint, lay it on the
table, and cover it over with a piece of glass of this shape; now draw
the outlines with a fine camel's hair pencil in black paint mixed with
varnish, and when this is dry, fill up the other parts with the proper
colors, shading with bistre also mixed with varnish. The transparent
colors are alone to be used in this kind of painting.
[Illustration]
TO EXHIBIT THE MAGIC LANTERN.
The room for the exhibition ought to be large, and of an oblong shape. At
one end of it suspend a large sheet so as to cover the whole of the wall.
The company being all seated, darken the room, and placing the lantern
with its tube in the direction of the sheet, introduce one of the slides
into the slit, taking care to invert the figures; then adjust the focus of
the glasses in the tube by drawing it in or out as required, and a perfect
representation of the object will appear.
EFFECTS OF THE MAGIC LANTERN.
Most extraordinary effects may be produced by means of the magic lantern;
one of the most effective of which is a
TEMPEST AT SEA.
[Illustration]
[Illustration]
This is effected by having two slides painted, one with the tempest as
approaching on one side, and continuing in intensity till it reaches the
other. Another slide has ships painted on it, and while the lantern is in
use, that containing the ships is dexterously drawn before the other, and
represents _ships in the storm_.
The effects of sunrise, moonlight, starlight, &c., may be imitated, also
by means of double sliders; and figures may be introduced sometimes of
fearful proportions.
[Illustration]
Heads may be made to nod, faces to laugh; eyes may be made to roll,
teeth to gnash; crocodiles may be made to swallow tigers; combats may be
represented; but one of the most instructive uses of the slides is to make
them illustrative of astronomy, and to show the rotation of the seasons,
the cause of eclipses, the mountains in the moon, spots on the sun, and
the various motions of the planetary bodies, and their satellites.
THE PHANTASMAGORIA.
Between the phantasmagoria and the magic lantern there is this difference;
in common magic lanterns the figures are painted on transparent glass,
consequently the image on the screen is a circle of light having figures
upon it; but in the phantasmagoria all the glass is made opaque, except
the figures, which, being painted in transparent colors, the light shines
through them, and no light can come upon the screen except that which
passes through the figure, as is here represented.
[Illustration]
There is no sheet to receive the picture, but the representation is thrown
on a thin screen of silk or muslin placed between _the spectators and
the lantern_. The images are made to appear approaching and receding by
removing the lantern further from the screen, or bringing it nearer to it.
This is a great advantage over the arrangements of the magic lantern, and
by it the most astonishing effects are often produced.
DISSOLVING VIEWS.
The dissolving views, by which one landscape or scene appears to pass into
the other while the scene is changing, are produced by using two magic
lanterns placed side by side, and that can be a little inclined towards
each other when necessary, so as to mix the rays of light proceeding from
the lenses of each together, which produces that confusion of images, in
which one view melts as it were into the other, which gradually becomes
clear and distinct.
HOW TO RAISE A GHOST.
[Illustration]
The magic lantern or phantasmagoria, may be used in a number of marvelous
ways, but in none more striking than in raising an apparent specter. Let
an open box, A B, about three feet long, a foot and half broad, and two
feet high, be prepared. At one end of this place a small swing dressing
glass, and at the other let a magic lantern be fixed with its lenses in a
direction towards the glass. A glass should now be made to slide up and
down in the groove C D, to which a cord and pulley should be attached, the
end of the cord coming to the part of the box marked A. On this glass the
most hideous specter that can be imagined may be painted, but in a squat
or contracted position, and when all is done, the lid of the box must be
prepared by raising a kind of gable at the end of the box B, and in its
lower part at E, an oval hole should be cut sufficiently large to suffer
the rays reflected from the glass to pass through them. On the top of the
box F place a chafing dish, upon which put some burning charcoal. Now
light the lamp G in the lantern, sprinkle some powdered camphor or white
incense on the charcoal, adjust the slide on which the specter is painted,
and the image will be thrown upon the smoke. In performing this feat the
room must be darkened, and the box should be placed on a high table, that
the hole through which the light comes may not be noticed.
THE THAUMATROPE.
This word is derived from two Greek words, one of which signifies
_wonder_, and the other _to turn_. It is a very pretty philosophical toy,
and is founded upon the principle in optics, that an impression made upon
the retina of the eye lasts for a short interval after the object which
produced it has been withdrawn. The impression which the mind receives
lasts for about the eighth part of a second, as may be easily shown by
whirling round a lighted stick, which if made to complete the circle
within that period, will exhibit not a fiery point, but a fiery circle in
the air.
THE BIRD IN THE CAGE.
Cut a piece of cardboard of the size of a penny piece, and paint on one
side a bird, and on the other a cage; fasten two pieces of thread, one on
each side, at opposite points of the card, so that the card can be made to
revolve by twirling the threads with the finger and thumb; while the toy
is in its revolution, the bird will be seen within the cage. A bat may in
the same manner be painted on one side of the card, and a cricketer upon
the other, which will exhibit the same phenomenon, arising from the same
principle.
[Illustration]
CONSTRUCTION OF THE PHANTASMASCOPE.
The above-named figure is a Thaumatrope, as much as the one we are about
to describe, although the term Phantasmascope is generally applied to the
latter instrument; which consists of a disc of darkened tin-plate, with
a slit or narrow opening in it, about two inches in length. It is fixed
upon a stand, and the slit placed upwards, so that it may easily be looked
through. Another disc of pasteboard, about a foot in diameter, is now
prepared and fixed on a similar stand, but with this difference, that it
is made to revolve round an axis in the center. On this pasteboard disc,
paint in colors a number of frogs in relative and progressive positions
of leaping; make between each figure a slit of about a quarter of an inch
deep; and when this second disc is made to revolve at a foot distance
behind the first, and the eye is placed near the slit, the whole of the
figures, instead of appearing to revolve with the disc, will all appear
in the attitudes of leaping up and down, increasing in agility as the
velocity of the motion is increased. It is necessary, when trying the
effect of this instrument, to stand before a looking-glass, and to present
the painted face of the machine towards the glass.
A very great number of figures may be prepared to produce similar
effects--horses with riders in various attitudes of leaping, toads
crawling, snakes twisting and writhing, faces laughing and crying, men
dancing, jugglers throwing up balls, &c.; all of which, by the peculiar
arrangement above detailed, will seem to be in motion. A little ingenuity
displayed in the construction and painting of the figures upon the
pasteboard disc will afford a great fund of amusement.
CURIOUS OPTICAL ILLUSION.
One of the most curious facts relating to the science of vision is
the absolute insensibility of a certain portion of the retina to the
impression of light, so that the image of any object falling on that point
would be invisible. When we look with the right eye, this point will be
about fifteen degrees to the right of the object observed, or to the right
of the axis of the eye, or the point of most distinct vision. When looking
with the left eye, the point will be as far to the left. The point in
question is the basis of the optic nerve, and its insensibility to light
was first observed by the French philosopher, Mariotte. This remarkable
phenomenon may be experimentally proved in the following manner:
Place on a sheet of writing-paper, at the distance of about three inches
apart, two colored wafers; then, on looking at the left-hand wafer with
the right eye, at the distance of about a foot, keeping the eye straight
above the wafer, and both eyes parallel with the line which forms the
wafers, the left eye being closed, the right-hand wafer will become
invisible; and a similar effect will take place if we close the right eye,
and look with the left.
ANOTHER.
Cut a circular piece of white paper, about two inches in diameter, which
affix to a dark wall. At the distance of two feet on each side, but a
little lower, make two marks; then place yourself directly opposite the
paper, and hold the end of your finger before your face, so that when the
right eye is open it shall conceal the mark on your left, and when the
left eye is open the mark on your right. If you then look with both eyes
at the end of your finger, the paper disc will be invisible.
ANOTHER.
Fix a similar disc of paper, two inches in diameter, at the height of your
eye on a dark wall; a little lower than this, at the distance of two feet
on the right hand, fix another of about three inches in diameter; now
place yourself opposite the first sheet of paper, and, shutting the left
eye, keep the right eye still fixed on the first object, and when at the
distance of about ten feet, the second piece of paper will be invisible.
Or, fix three pieces of paper against the wall of a room, at equal
distances, at the height of the eye. Place yourself directly before them,
at a few yards distance, close your right eye, and look at them with your
left, when you will see only two of them, suppose the first and second;
alter your position a second time, and you will see the second and third,
but never the whole three pieces together.
On a sheet of black paper, or other dark ground, place two white wafers,
having their centers three inches distant. Vertically above the paper,
and to the left, look with the right eye, at twelve inches from it, and
so, that when looking down on it, the line joining the two eyes shall be
parallel to that joining the center of the wafers. In this situation close
the left eye, and look full with the right perpendicularly at the wafer
below it, when this wafer only will be seen, the other being completely
invisible. But if it be removed ever so little from its place, either to
the right or left, above or below, it will become immediately visible, and
start, as it were, into existence. "It will cease to be thought singular,"
says Sir John Herschel, "that this fact of the absolute invisibility of
objects in a certain point of the field of view of each eye should be one
of which not one person in ten thousand is apprized, when we learn that
it is not extremely uncommon to find persons who have for some time been
totally blind with one eye without being aware of the fact."
THE PICTURE IN THE AIR.
One of the numerous optical illusions which have, from time to time,
been evolved by scientific minds, is that of making an image or picture
appear in the air. This is produced by means of a mirror, and an object in
relief, upon which a strong light is thrown--the mirror being set at such
an angle as to throw up the reflection of the image to a certain point,
in the view of the spectator. This illusion is produced as follows: Let
a screen be constructed in which is an arched aperture, the center of
which may be five feet from the floor; behind the screen is placed a large
mirror of an elliptical form. An object is now placed behind the screen,
upon which the light of a strong lamp is thrown from a point above the
mirror, and is received by the mirror and reflected to the center of the
arched cavity in the screen, where it will appear to the spectator. Care
should be taken to place the image in an inverted position, and the light
should be so placed that none of it may reach the opening: the light must
also be very powerful.
BREATHING LIGHT AND DARKNESS.
[Illustration]
The following experiment, if performed with care, is exceedingly striking.
Let S be a candle, whose light falls at an angle of 56° 45´ upon two plate
glasses, A B, placed close to each other; and let the reflected rays, A C,
B D, fall at the same angle upon two similar plates, C D, but so placed
that the plane of reflection from the latter is at right angles to the
plane of reflection from the former. An eye placed at E, and looking at
the same time on the two plates, C and D, will see very faint images of
the candle S; which by a slight adjustment of the plates, may be made
to disappear almost wholly, allowing the plate C to remain where it is.
Change the position of D till its inclination to the ray B D is diminished
about 3°, or made nearly 53° 11´. The distance may be easily found by a
little practice. When this is done, the image that had disappeared on
looking into D, will be restored, so that the spectator at E, upon looking
into the two mirrors, C D, will see no light in C, because the candle has
nearly disappeared, while the candle is distinctly seen in D. If, while
the spectator is looking into these two mirrors, either he or another
person breathes upon them gently and quickly, the breath will revive the
extinguished image in C, and will extinguish the visible image in D.
EXPLANATION.--The light A, C, B, D is polarized by reflection from the
plates A B, because it is incident at the polarizing angle 56° 45´ for
glass. When we breathe upon the plates C D, we form upon their surface
a thin film of water, whose polarizing angle is 53° 11´, so that if the
polarized rays A C, B D, fall upon the plates C D, at an angle of 53° 11´,
the candle from which they proceed would not be visible, or they would not
suffer reflection from the plates C D. At all the other angles the light
would be reflected, and the candle visible. Now the plate D is placed at
an angle of 53° 11´, and C at an angle of 56° 45´, so that when a film of
water is breathed upon them, the light will be reflected from the latter,
and none from the former; that is, the act of breathing upon the glass
plates will restore the invisible, and extinguish the visible image.
TO SHOW WHAT RAYS OF LIGHT DO NOT OBSTRUCT EACH OTHER.
[Illustration]
Make a small hole in a sheet of pasteboard A, and placing it upright
before three candles B, placed closely together, it will be found that the
images of all the candle flames will be formed separately on a piece of
paper C, laid on the table to receive them. This proves that the rays of
light do not obstruct each other in their progress, although all cross in
passing through the hole.
HOW TO SEE THROUGH A PHILADELPHIA BRICK.
Construct a hollow box or case, like the figure in the margin. One side is
purposely removed in the engraving, to enable you to see the arrangement
of the interior, A, B, C, and D, are four small pieces of looking-glass,
all placed at an angle of 45°, with respect to those sides of the box on
which they are fixed; at E and G, two flat pieces of glass are inserted,
as in the eye-glass of a telescope. Supposing you look through the opening
E, in the direction of an object placed at O, you would see it in the
same manner as if there was an uninterrupted view between E and G, which
is evidently not the case. The cause of this is readily explained. The
image of the object at O is received on the looking-glass A, by which it
is reflected to B, as it is again from B to C, and afterwards to D; and
this last image in D is seen by the eye of the spectator placed at E, in
the same direction as if in reality he was looking at the real object
itself, in the direction of the dotted line from O to E. From this it is
evident that the placing an opaque body at F, cannot prevent the object
at O being seen. Of course all this arrangement of the instrument is
concealed, and you place it in the hands of a companion, that he may look
through E or G, it matters not which, at any object placed beyond. You may
then safely lay a wager that your instrument is of so magical a nature
that it will enable you to see through a brick wall; but as a single brick
will be more convenient, and equally wonderful, you are willing to satisfy
his doubts at once. Of course, the hand or the hat, or any other opaque
object, will answer the same purpose.
[Illustration]
THE STEREOSCOPE.
This is one of the newest and most interesting optical surprises invented,
but, like many other instruments, it is indulged with a very hard name,
which means, "Solids I see." For mere amusement, the instrument supplied
with the necessary pictures may suffice, and they are now sold like the
kaleidoscope at all optical instrument makers, toyshops, &c. The effect
consists in obtaining the perfect solidity of a geometric object from two
ordinary drawings, pictures of columns, statuary, figures, flowers, &c.,
&c., having a rounded appearance, "breadth," and keeping, which induce
the spectator to believe he is gazing at the natural figure. Our limits
preclude a lengthened description of the philosophy of this instrument,
the invention of Professor Wheatstone. We may, however, first recommend
our readers to study the structure of the eye, in Brewster's Treatise
on Optics, which may be thoroughly impressed on the mind by dissecting
carefully the eye of a sheep or bullock. Now, if we cut open a portion of
the eye of a recently killed animal, and look in upon the retina, which is
a delicate network of nerves, and is considered the "mind of the eye," we
shall behold all images inverted. How then, do we see them upright? Again,
as we have two eyes at a distance from each other, the images formed on
the two retinæ cannot be precisely alike: how is it that confusion is not
the result, instead of perfect images, in which we can appreciate the
geometric niceties of length, breadth, and thickness? Now the stereoscope
assists us in understanding these difficult questions; and, quoting
Professor Wheatstone, we find he states "that the theory which has
obtained the greatest currency is that which assumes that an object is
seen single because its pictures fall on corresponding points of the two
retinæ; that is, on points which are similarly situated with respect to
the two centers, both in distance and position. This theory supposes that
the pictures projected on the retinæ, are exactly similar to each other
corresponding points of the two pictures falling on corresponding points
of the two retinæ."
Now, the fact is, that an object presents an entirely different appearance
to each eye. Sir D. Brewster remarks, "That were a painter called upon to
take drawings of a statue, as seen by each eye, he would fix at the height
of his eyes a metallic plate, with two small holes in it, and he would
then draw the statue as seen through the holes by each eye." With the
utmost care, however, he could not reproduce the statue by their union.
In order to do this, a camera with _two lenses_ of the same aperture and
focal length, placed at the same distance as the _two eyes_, must be
constructed and used.
The stereoscope is, consequently, an imitation of the powers of the eyes,
giving solidity and a perfect relievo appearance to any two pictures
which might be drawn separately from the two lenses in the camera obscura
mentioned. That is to say, if it were possible to be behind the retina of
each eye, and draw the two pictures of any object seen by our eyes, those
pictures put into the stereoscope, would reproduce the solidity from
which they were drawn.
Two instruments are sold, and may be obtained with the photographic
pictures, almost at any optician's, viz.: the reflecting and the
refracting stereoscope, of which we give drawings.
[Illustration: _Fig. 1_
THE REFLECTING STEREOSCOPE.]
[Illustration: _Fig. 2_
REFRACTING STEREOSCOPE.]
_a_ _a_, the two pictures, B B, the two mirrors, so adjusted that their
backs form an angle of ninety degrees with each other, i. e., the quarter
of a circle.
OCULAR SPECTRA.
One of the most curious affections of the eye is that in virtue of which
it sees what are called _ocular spectra_, or accidental colors. If we
place a red wafer on a sheet of white paper, and, closing one eye, keep
the other directed for some time to the center of the wafer, then, if
we turn the same eye to another part of the paper, we shall see a green
wafer, the color of which will continue to grow fainter and fainter, as we
continue to look at it.
By using differently-colored wafers, we obtain the following results:
WAFER. SPECIMEN.
Black, White.
White, Black.
Red, Bluish Green.
Orange, Blue.
Yellow, Indigo.
Green, Violet, with a little Red.
Blue, Orange Red.
Indigo, Orange Yellow.
Violet, Bluish Green.
BRILLIANT WATER MIRROR.