REFRACTING OPTICAL INSTRUMENTS.
I. _The Magic Lantern._
No other optical instrument has ever caused so much wonderment and
delight, from its origin to the present time, as this simple
contrivance. For a long time its true value was overlooked, and only
ridiculous or comic slides painted, but its educational importance is
now being thoroughly appreciated, not only on account of the size of the
diagrams that may be represented on the disc, but also from the fact
that the attention of an audience is better secured in a room when the
only object visible is the diagram under explanation. The lenses it
contains are a "bull's eye" or plano-convex, nearest the light, and a
double convex glass, for the purpose of focussing the picture which is
inverted and placed between the two lenses. (Fig. 293.)
[Illustration: Fig. 293. The magic lantern.]
In many books full directions are given for painting the glass slides,
but this is an art that requires very great practice and experience. A
person may know how to draw and paint on paper or canvas, but it is
quite a different thing where glass is concerned, and unless the
juvenile artist has taken lessons from a regular painter on glass, his
or her efforts are likely to be very unsatisfactory. In many popular
works embracing the subject of optics, full directions are given on the
mode of painting the slides for the magic lantern, or dissolving views;
a new era, however, has dawned upon this mode of illustration, in the
preparation of photographs on glass of the most lovely description, and
now instead of exhibiting mere daubs of weak colouring, photographic
pictures of singular perfection can be procured of Messrs. Negretti and
Zambra, Holborn, who have turned their attention especially to this
branch, and supply slides of all sizes.
II. _The Dissolving Views._
This very pleasing modification of the ordinary magic lantern is
displayed with the assistance of two lanterns of the same size, provided
with lamps and lenses which are exactly alike. They are best arranged
[Page 304] on one board, side by side, and if kept parallel with each
other, the circles of light thrown from the two lanterns would not
coincide on the screen; it is therefore necessary to place one of them
at an angle which will vary according to the distance from the screen.
The task of making the two circles of light overlap each other precisely
on the disc, is called centering the lanterns, and is the first thing
that must be attended to before exhibiting the slides. The slides for
the dissolving views are all painted of the same size, and supposing a
scene such as a church with a bridal procession and the trees in full
foliage, to represent summer, is first thrown on to the disc, it may be
changed to winter by putting another picture of the same subject, but
painted to represent bare trees, and the church and ground covered with
snow, and a grave open, with a funeral procession. The two pictures must
not be projected on the screen at the same time, and here the dissolving
mechanism is required; it consists of two fans so arranged that they may
be raised or lowered by a rack-work and handle; one fan in descending
covers one of the nozzles of the lanterns, and the other leaves the
second lantern open, and free to project the picture; the dissolving is
managed by slowly moving the handle of the rack-work, so that one
quarter of the picture already on the disc is cut off, and one quarter
of the new one thrown on. As the movement proceeds, one half of the old
picture is shut out, and one half of the new slide takes its place, and
so on, till the whole of the original picture is cut off by the fan and
the new one comes into view, and it is in this way the effect of the
change from summer to winter is produced. (Fig. 294.)
[Illustration: Fig. 294. Nozzle of one lantern, with the fan, A, raised,
and in the position to throw a picture on the disc. B. The other fan
shutting off the second lantern.]
When two pictures such as those already described, dissolve one into the
other, of course the same building or other marked portion of the
subject, must strictly coincide in each picture on the disc, or else the
two pictures are apparent, and the illusion is destroyed. The pictures
must all be centered before the exhibition commences. By the arrangement
of Mons. Duboscq, one electric light serves to illuminate both lanterns
by making use of mirrors. The dissolving apparatus is likewise very
[Page 305] simple, and consists of two diamond-shaped openings in a
brass frame, which open and shut alternately by a slide worked with a
handle. The single light is not to be recommended, as it is somewhat
troublesome to manage properly. (Fig. 295.)
[Illustration: Fig. 295. A. The electric light. B B. The two sets of
lenses for the two pictures. C. The dissolving mechanism. D. The picture
on screen.]
When dissolving views are required on a grand scale, the lenses must be
exceedingly large, and the condenser (corresponding with the
"bull's-eye" of the simple magic lantern) should be at least nine or
eleven inches in diameter, and the front glasses must be of a superior
make. The lenses for a large lantern lit by the oxy-hydrogen light, are
arranged as in the next cut. (Fig. 296.)
[Illustration: Fig. 296. A. The lime light. B. The condensers. C. The
picture. D D. The front lenses for focussing, with rack-work.]
At the Polytechnic the author had no less than six lanterns working at
or about the same time, to produce effects, in the views illustrating
the voyages of Sinbad the Sailor; and in order to obtain the increased
[Page 306] results required for dioramic effects, such for instance as
the Siege of Delhi, showing the bursting of the shells, &c., the four
fixed lanterns (the fronts of which are shown in the next cut) were
always employed. The two upper lanterns are dissolved by discs of brass
worked by the hand, and the lower ones with the fans. (Fig. 297.)
[Illustration: Fig. 297. Fronts of the four lanterns, showing how the
dissolving mechanism is arranged.]
"Behind the scenes" always has a great attraction for young people; we
have, therefore, in the frontispiece, with the help of Mr. Hine (who
painted a great number of the photographs shown at the Polytechnic
during the author's management), given a section of the large theatre
taken whilst the effective scene of the Siege of Delhi was in progress.
The optical effects were assisted by various sounds in imitation of
war's alarms, for the production of which, more _volunteers_ than were
required would occasionally trespass behind the screen, and produce
those terrific sounds that some persons of a nervous temperament said
were really stunning. In a page picture, we have also given a correct
drawing of the interior of the optical box at the Polytechnic, with the
four fixed lanterns, and side cupboards to hold the glass pictures. The
four lanterns worked on a railway, with wheels and a circular
turn-table; they could be removed, and the microscope arranged in their
places.
[Illustration: Before and behind the screen at the Polytechnic during
the exhibition of the dioramic effects of the siege of Delhi. _p. 306_]
[Page 307]
III. _The Oxy-Hydrogen Microscope._
Many persons will recollect the first exhibition of this instrument in
Bond-street, by Mr. J. T. Cooper, and Mr. Cary, succeeded by the
Adelaide Gallery exhibition of scientific wonders and an oxy-hydrogen
microscope. The apparatus for this purpose consists of three condensing
lenses and an object glass. The objects, such as live aquatic insects,
are placed in glass troughs containing water; the other objects, ferns,
feathers, butterflies, algæ, &c. &c., being mounted on slides in the
ordinary way with Canada balsam. (Fig. 298.)
[Illustration: Fig. 298. A. The lime light. C C C. Condensers. D. The
object, such as a tank of water containing live insects. E. The object
glasses.]
IV. _The Physioscope._
This instrument, brought out at the Polytechnic during the time that Mr.
J. F. Goddard managed the optical department of the institution, always
excited the greatest mirth and astonishment amongst the numerous
visitors; and _habitués_ of the old place may remember the good-natured
inimitable maudlin simper with which poor Mr. Tait (who was one of the
living objects shown on the disc) used to drink off the glass of wine
and then wink at the audience. When we say Mr. Tait used to wink, of
course it is understood that he was personally invisible, and his
apparition or image only appeared on the disc. The countenance is
brilliantly illuminated by the oxy-hydrogen light, and being placed near
the lenses, the rays are reflected from the face into the physioscope,
and being properly focused, and the inversion of the image corrected,
the perfect representation of the human countenance is apparent on the
disc. The lenses and concave reflectors required are shown in the
section of the physioscope. Messrs. Carpenter and Westley, of
Regent-street, have brought the manufacture of magic lanterns to great
perfection; and Mr. Collins, of the Polytechnic, constructs every kind
of dissolving view apparatus, oxy-hydrogen microscopes, physioscopes,
&c. (Fig. 299.) With this instrument any opaque objects (provided they
reflect light properly) may be displayed to a large audience. Plaster
casts appear with singular beauty and softness, whilst flowers, stuffed
birds, and especially humming birds, are excellent objects for the
physioscope.
[Page 308]
[Illustration: Fig. 299. A. One or more lime lights, throwing rays
reflected by concave mirrors on to the face B, from whence they are
reflected to C C, the first condensers. D D. Object glasses. This
instrument is made by Mr. Collins, who has the tools for making the
reflectors with correct curves. The picture of the face on the disc is
covered with black spots if the reflectors are not perfect.]
V. _The Camera Obscura._
A "dark chamber" is the name of a most amusing, and now, in the improved
form, extremely valuable instrument for photographic purposes. It is
occasionally to be met with in public gardens, and there is a very good
one on the Hoe at Plymouth. The construction of the camera for observing
the surrounding country is very simple, and merely consists of a flat
mirror placed at an angle, by which the picture is reflected through a
double-convex lens on to a white table beneath. (Fig. 300.)
[Illustration: Fig. 300. A. The mirror. B. The convex lens. C. The white
table.]
[Page 309]
The term "focusing," or the art of moving the lenses so that a sharp
image may be obtained, has been frequently mentioned in this article,
and perhaps it may be as well to describe the mode of ascertaining the
focal distance of a lens by experiment.
Hold the lens opposite the window so that a bright picture of the
window-sash may be obtained on a sheet of paper pinned against the wall,
and the distance of the lens from the paper will be the focal length.
If the lens has a very long focal length, it may be determined as
follows:--Measure the distance between the lens and the object, and also
from the image; multiply these distances together, and divide the
product by their sums; the quotient will give the focal distance.
VI. _The Decomposition of Light--"its Analysis and Synthesis."_
It is in the Italian language that the bride, the emblem of purity, is
called Lucia (_Lux_, light); and surely if an illustration were required
of beauty and singleness, light would be named poetically as
appropriate; but physically it is not of a single nature, it is
composite, and made up of seven colours. The instrument required to
refract a ray of light sufficiently to break it into its elementary
colours is called the prism, and is a solid having two plane surfaces,
called its refracting surfaces, with a base equally inclined to them.
(Fig. 301.)
[Illustration: Fig. 301. The prism. The base, A B, is equally inclined
to the refracting surfaces, C A, C B.]
It was in 1672 that Sir Isaac Newton made his celebrated analysis of
light, by receiving a sunbeam (as it passed through a hole in a shutter)
on to the refracting surface of a prism, and throwing the image or
spectrum on to a screen, where he observed the seven colours, red,
orange, yellow, green, blue, indigo, and violet, and thus proved "_that
there are different species of light, and that each species is disposed
both to suffer a different degree of refrangibility in passing out of
one medium into another, and to excite in us the idea of a different
colour from the rest; and that bodies appear of that colour which arises
from the composition of those colours the several species they reflect
are disposed to excite_."
Sir Isaac Newton's name would have been immortalized by this discovery
alone, even if he had not possessed that transcendent ability which
raised him above all other mathematicians and physicists. It is at the
same time interesting to know that the ancient author Claudian (A.D.
420) inquires "whether colour really belongs to the substances
themselves, or whether by the reflection of light they cheat the
eye--_enquires sitve color proprius rerum, lucisne repulsa eludant
aciem_."
Sir Isaac Newton determined that the spectrum could be divided into 360
equal parts, of which red occupied 45, orange 27, yellow 48, green 60,
blue 60, indigo 40, violet 80. He also discovered that if the highly
refracted rays, the seven colours, or spectrum were received into [Page
310] a concave mirror or a double-convex lens, that they again united
and formed white light. In order to demonstrate the properties of the
prism in various positions, the next diagram may be adduced. (Fig. 302.)
[Illustration: Fig. 302. A. The ray of light passing through two prisms
B placed base to base. In this position the light passes through to the
second prism, C, without alteration. At C the decomposition of light
occurs, and the spectrum is shown at D D. The top prism at B used singly
would reflect the ray to E without decomposing it into the coloured
rays.]
The rainbow is the most beautiful natural optical phenomenon with which
we are acquainted; it is only seen in rainy weather when the sun
illuminates the falling rain, and the spectator has the sun at his back.
There are frequently two bows seen, the interior and exterior bow, or
the primary and secondary, and even within the primary rainbow, and in
contact with it, and outside the secondary one, there have been seen
other bows beyond the number stated.
The primary or inner rainbow consists of seven different coloured bows,
and is usually the brightest, being formed by the rays of light falling
on the upper parts of the drops of rain. The exterior bow is formed by
the rays of light falling on the lower parts of the drops of rain; and
in both cases the rays of light undergo refraction and reflection, hence
the opinion of Aristotle, that the rainbow is caused only by the
reflection of light, is not correct.
The first refraction occurs when the rays of light enter, and the second
when they emerge from the spheroids of water in the first bow; the
refracted rays undergo only one reflection, whereas in the second the
brilliancy of the colours is impaired by two reflections.
The spectrum from the electric light is one of the most gorgeous
exhibitions of colour that can be conceived; and the instruments
required for the purpose are illustrated in No. 1 (Fig. 303), whilst
the [Page 311] synthesis of the coloured rays and production of white
light is shown at No. 2 of the same figure. (Fig. 303.)
[Illustration: Fig. 303. No. 1. A. The electric light. B. The narrow
slit through which the light passes to the convex lens, C. D. The prism.
E. The spectrum. No. 2 is the same for A B C D; but F is the convex lens
collecting the scattered rays, and forming white light at G.]
VII. _Duration of the Impression of Light._
If a circular disc is painted with the prismatic colours taken in the
same proportion with respect to each other in which they are exhibited
in the spectrum made by the prism, and the wheel is turned swiftly, then
the individual colours disappear, and nearly white light is apparent.
The cause is due to the same principle that creates the appearance of a
complete circle of fire when a burning squib is moved quickly round
before it is thrown away to burst, and as it is evident that the burning
squib cannot be in every part of the circle at the same moment, there
must be some inherent faculty belonging to the human eye which enables
it to retain for a definite period the impression of images that may
fall upon it; and this principle has been so far pressed, as it were,
beyond its limits, that it is gravely asserted the image of a man's
murderer "might be discovered on the retina of the eye-ball if that
could be examined sufficiently quick after death." The fixture of the
picture is said to be due to a sort of natural photographic process; but
such fanciful statements often lead the mind into dream-land only, and
so we will return to the fact of the duration of the impression of light
on the eye as evidenced by several ingenious optical instruments, and
especially by the scientific inventions of Dr. Faraday, Dr. Paris, and
of Mr. Thomas Rose of Glasgow.
By careful experiment M. D'Arcy found that the light of a live coal
moving at the distance of 165 feet, maintained its impression on the
[Page 312] retina during the seventh part of a second. Hence the cause
of the recomposition of white light when the colours on the disc are
quickly rotated. Each colour at any point succeeds the other before the
impression of the last is gone from the eye, and provided the colours
move round within the seventh part of a second, they are all impressed
together on the eye, and meeting on the retina, produce the effect of
white light.
VIII. _The Phenakistiscope._
This amusing instrument consists of a turning wheel upon which figures
appear to jump, walk, or dance. The disc or wheel is of cardboard, upon
which are painted (towards the periphery) figures in eight, ten, or
twelve postures. Thus, if it is desired to represent clowns turning
round in a circle, twelve different positions of the figure in the act
of turning are painted on the disc, and above each of the figures on the
wheel a slit is cut about one inch long, and a quarter of an inch wide
in a direction corresponding with the radii of the circle. This simple
form of the instrument is used by placing the figured side towards a
looking-glass and then causing it to revolve at a certain speed, which
is ascertained by experiment; and as the spectator looks through the
slits into the looking-glass, the clowns appear to turn round. At the
Polytechnic Institution there are two of these wheels with
looking-glasses, and although the same designs have done duty for many
years, they still attract the public attention. (Fig. 304.)
[Illustration: Fig. 304. Design for the phenakistiscope. The spectator
is supposed to be looking towards a mirror through the slits. It is
supported by a handle through the centre, round which it is twirled by
the other hand.]
In the "Journal of the Royal Institution" Mr. Faraday has described some
very interesting experiments and optical illusions produced by the
revolution of wheels in different directions and velocities. The wheels
are made of cardboard, and by cutting out two cog wheels of an equal
size, and placing one above the other on a pin, the usual hazy tint when
the cogs are acting is apparent when they are whirled round; but if the
two cog wheels are made to move in opposite directions, there will be
the extraordinary appearance of a fixed spectral wheel. If the cogs are
cut in a slanting direction on both wheels, the spectral wheel will
exhibit slanting cogs; but if one wheel is turned so that the cogs shall
point in opposite directions, then the spectral wheel will have straight
cogs. A number of such wheels set in motion in a darkened room, and
illuminated suddenly with the light from the electric spark, appear to
stand perfectly still, although moving with a great velocity. An
expensive instrument has been constructed by Duboscq, for the [Page
313] purpose of showing the usual phenakistiscope effects on the screen
with the magic lantern; a very limited picture, however, is shown, and
there is still great room for the improvement of the apparatus. (Fig.
305.)
[Illustration: Fig. 305. Phenakistiscope made by Duboscq, of Paris. No.
1. Apparatus in elevation with the condensers. No. 2. Section of the
apparatus. A. The light. B. Condenser, or plano-convex lens. C. Round
glass disc with design painted on it. D. Wooden disc with four
double-convex lenses placed at equal distances from each other, so as to
coincide with C, whilst rotating. Both the latter and C rotate, and the
picture is focussed on the disc by the lenses F. No. 3. Glass plate,
with device painted thereon.]
[Page 314]
IX. _The Thaumatrope._
This very simple toy was invented by the late Dr. Paris, who gave it an
appropriate name, compounded of the Greek words, [Greek: _thauma_],
wonder, [Greek: _trepô_], to turn. The duration of the impressions of
light on the eye is very apparent whilst using this toy, which is
usually made of a circular piece of cardboard, having on one side a
painting of a man's head, and on the other a hat; or a picture of a
lighted candle on one face of the cardboard, and an extinguisher on the
other; or a gate, and a horseman leaping it. Each pair of designs
painted on opposite sides of the cardboard appear to be one when twisted
round by strings tied to the opposite edges of the cardboard circle. The
_rationale_ of this experiment being, that the picture of one
design--such as the head and face--is retained by the eye until the hat
appears, and being mutually impressed upon the nerve of vision at very
nearly the same instant of time, they appear as one picture.
X. _The Kalotrope._
This is an optical arrangement by Mr. Thomas Rose, of Glasgow, primarily
designed for showing the illusions of the phenakistiscope and kindred
devices to a numerous audience; but more remarkable for its
presentations of very beautiful spectra, composed of the multiplication,
combination, and involution of simple figures disposed around a disc.
The arrangement consists of a movement for giving considerable velocity
to two concentric wheels, working nearly in contact, and moving in
contrary directions. But the only part of the apparatus that requires
special explanation and illustration is the device disc and the disc of
apertures; the first of which is placed on the hinder wheel, and the
second on the front wheel. We give figures of the two discs, premising,
however, that each is capable of an almost infinite variety of
characters. No. 1 (Fig. 306) presents in its four quadrants the
perforations for four distinct discs of apertures; and No. 2 is a device
disc, consisting of twelve equidistant black balls. Under _a_ the balls
will be presented as twenty-four ovals; under _b_, as forty-eight
involved figures, beautifully variegated; under _c_, as an elaborate
lacework; and under _d_, as a rich variegation of form and colour. Every
fresh disc of devices and disc of apertures of course opens up a new
field of effect. Thus, if we take a disc bearing twelve repeats of a
ball in the interior of a ring, each repeat being so painted that its
position is advanced in the ring until it reaches in the twelfth ring
the point whence it started, and place this on the back disc of the
kalotrope, having previously removed the first one, no effect is
observed when the wheel is rotated beyond the spreading out of the
design and general appearance of hazy black circles. When, however, the
disc, with twelve slits or apertures, is now placed on the front wheel,
and the two rotated in opposite directions, then the whole figure starts
as it were into existence, and each ball apparently moves round the
interior of its circle. [Page 315] The apparatus was produced at the
Royal Polytechnic Institution by the author, and excited much interest.
(Fig. 306.)
[Illustration: Fig. 306. Nos. 1 and 2 are the discs. No 3. Kalotrope in
elevation. No. 4. Side view of kalotrope, showing the multiplying wheels
and the perforated and painted discs moving in opposite directions.]
XI. _The Photodrome._
This is a second optical arrangement by Mr. Rose for showing spectral
illusions; and it is superior to the last, inasmuch as it offers to the
public lecturer a most effective means of presenting these deceptions to
a large audience. It differs from the kalotrope in several important
points. It dispenses with the discs of apertures, and leaves the device
disc with its face fully exposed to the spectators. The effects are
produced by a powerful light, thrown through the tube of a lantern, and
broken by a wheel working across it. The apparatus, as it at present
stands in the inventor's possession, consists of two distinct parts; the
one a movement for the device discs, and the other for the light. A
wheel four feet in diameter is connected with a train of movement
capable of giving it five hundred or six hundred revolutions per minute.
On this wheel the device disc is placed, in full view of the spectators,
and set in motion. From an opposite gallery the light is thrown, and
[Page 316] broken by a wheel of such diameter and number of apertures as
will admit the velocity of the _photodrome_ (or light-runner) to be at
least _six_ times the velocity of the device disc; whilst the apertures
are of such width as to restrict the duration of the light-flash to
about one-two-thousandth of a second. The wheel working across the light
has a train of movement for raising the velocity to two thousand
revolutions per second. The management of the apparatus is very simple.
The device-wheel is brought to a steady, rapid rotation, and the
operator on the light then works his wheel with gradually increasing
velocity, until he overtakes the figures of the device, where, by mere
delicacy of touch, he is able to hold them stationary or give them
motion, at pleasure.
Theories of light and colour still agitate the scientific world,
although that man must be bold who will assert that his hypothesis is
fitted to explain every difficult point that arises as our experimental
knowledge increases. Mr. G. J. Smith, of the Perth Academy, has
propounded a very ingenious theory of light and colour, supported by
some clever experiments. But, as Solomon says, "there is nothing _new_
under the sun," and in an able paper Mr. Rose, of Glasgow, lays claim to
the anticipations of Mr. Smith's theory as follows:--
"My attention has been directed to a paper entitled 'The Theory of
Light,' by G. John Smith, Esq., M.A., of Perth Academy. I think it is
now nearly two years since I communicated an interesting fact to
Professor Faraday, and to a member of our local Philosophical
Institution, which may fairly claim to have anticipated Mr. Smith's
theory. The fact was this: that if a piece of intensely white card be
held in one hand, with the light of a powerful gas-jet falling upon it,
and if the other hand has command of the gas-tap, as the light is
gradually reduced, the card will assume the prismatic colours down to
intense blue, and as the light is restored the colours will present
themselves in inverse order. The experiment showed, very conclusively to
my mind, that light is homogeneous, and that what we name colour is only
the various affection of the optic nerve by a greater or lesser
radiation of light from a focal point in an imperfect reflector--say, in
the instance, a white card. I apprehend that Mr. Smith confuses his
theory when he speaks of alternations of light and shadow producing
colour. Shadow, or darkness, is mere negation of light. We do not see
mixtures of light and darkness, or blackness and whiteness, but light in
its several degrees of intensity. Mr. Smith's experiments present only
what my kalotrope has done, and what my later device, the photodrome
(now nearly three years old) is doing in a much more perfect manner. It
is one of the mysteries intelligible only to the initiated, that whilst
Mr. Smith's paper seems to have been received with great favour by the
British Association, my communication relative to the photodrome was
voted 'not _sufficiently practical_.'
* * * * *
"Since I have come before the public with an experiment, which in any
view is an interesting one, permit me to reproduce it under several
distinct conditions, and to add a brief narrative of remarkable
presentations of colour that have come before me, and which, so far as I
am [Page 317] aware, are perfectly novel, or known only through the
more recent experiments of Mr. Smith. Professor Faraday very courteously
acknowledged my communication of the experiment with the card, but said
that it only partially succeeded with him, and added that probably this
was owing to some decay of sensitiveness in his eyes. More likely I
failed to state with sufficient clearness the conditions of the
experiment, since I have always found nine persons out of ten perfectly
agreed as to the effects produced when they have been at my side. The
transitions from white to yellow, orange, red, and thence to intense
blue, are, I may say, invariably admitted. Success depends on a very
slow and regular reduction and restoration of the light. I have given
one method of performing the experiment, and will add other two. Allow
the light to remain undisturbed, and begin by holding the card near to
it; then keep the hand steady and the eye intently fixed upon the card,
and retire gradually with your back to the light, and the colours will
change in the order of the prismatic spectrum from yellow to intense
blue. On returning backwards towards the light the colours will again
present themselves, but in inverse order. In this form of the experiment
we are certain that the light remains precisely the same throughout. The
third method is this: Place a circle of white card, about three inches
in diameter, in the centre of a black board, and let a spectator stand
within twelve inches of the board, with his eyes fixed upon the card.
Let an operator be provided with a light so covered that it shall not
fall on the eye of the spectator; then, as he retires with the light or
returns with it, the spectator will see the colours as before. This
arrangement evidently subjects the experiment to a severe test, since
the black board enhances the whiteness of the card, and tends to
preserve it.... Whilst pursuing my principal object, I frequently
noticed most remarkable presentations of colour; but, as the conditions
were for the most part unsuitable to the lecture-room, I gave them only
a passing regard. Allow me to instance a few of the experiments.
"The first refers to the kalotrope, which may be briefly described as an
arrangement of two concentric wheels, working nearly in contact and in
contrary directions. Discs of various devices are provided for the
hinder wheel, and a number of perforated black discs for the one in
front. When a disc charged with twelve _black_ radii is placed on the
hinder wheel, the six spokes of the front wheel, in passing rapidly
across it, convert the twelve black radii into twenty-four apparently
stationary _white_ radii upon a tinted ground. Here is a remarkable
presentation of the complementary, inasmuch as it is placed permanently
before the eye by persistence.
"The second experiment is performed with the photodrome, which consists
of an independent wheel to receive the device discs, and an apparatus
(altogether apart, and, if desired, out of sight) by which flashes of
light are thrown upon the disc in rapid and regular succession. Now, if
a disc charged with twelve dark blue balls, nearly in contact, be placed
upon the wheel, and a little natural light be allowed to fall [Page
318] upon it, so soon as it is thrown into rapid revolution, and flashes
of artificial light (insulated in a lantern) are duly measured out upon
it, we see twelve apparently stationary light-blue balls upon a zone of
bright orange. Here, again, there is nothing for which we are not
prepared; the complementary is suddenly presented, and it is maintained
permanently before the eye by persistence.
"A third experiment may prove interesting in its relation to Mr. Smith's
ingenious theory. Place the kalotrope opposite a bright northern noonday
sky, remove the front wheel, and affix to the hinder wheel one of the
perforated black discs used for the kalotropic effects. The
experimentalist stands at the back of the instrument, and can see the
sky only through the apertures in the black disc. Cause these apertures
to pass the eye at intervals varying from one-half to one-sixth of a
second, and very remarkable presentations of colour are seen. Under the
lower velocities the sky flashes, and assumes an unnatural brilliancy,
and the intervals of the fourth and fifth of a second give it sometimes
a crimson, at others a deep purple colour. Now, what are we to infer
from this experiment? Certainly _not_ that the pulsations have
absolutely produced variety of colour. At every pulsation the full
natural light falls upon the eye, and the intervals between the
pulsations give time for the reaction necessary to the suggestion of
complementary colour, and that under manifold modifications arising out
of the ever-changing condition of the eye during the experiment. If the
apertures pass the eye with a velocity exceeding one-sixth of a second,
the effect ceases. There is then perfect persistence, and the eye
apprehends nothing but the ordinary light of the sky, reduced in
intensity, with nothing to break its uniformity or give it a chromatic
character.
"A fourth experiment is kindred to the last. Place the kalotrope under
the same adjustment and management as before, in front of a brilliant
sunset, and the spectator will see, with more than a poet's vision,
'The rich hues of all glorious things.'"
XII. _The Kaleidoscopic Colour-top._
This invention by John Graham, of Tunbridge, is designed to show that
when white or coloured light is transmitted to the eye through small
openings cut into patterns or devices, and when such openings are made
to pass before the eye in rapid successive jerks, both form and colour
are retained upon the nerve of the visual organ sufficiently long to
produce a compound pattern, all the parts of which appear
simultaneously, although presented in succession. The instrument forms,
therefore, a pleasing illustration of the law that the eye requires an
almost inappreciably short space of time to receive an impression, and
that such impression is not directly effaced, but remains for an
assignable though very limited period. The results are obtained by
rotating two discs on a wheel, the lower disc containing colours, and
the upper one the [Page 319] openings; this latter disc is made to
vibrate as well as to rotate, thus allowing the eye to receive the
coloured light reflected from below, which light assumes, at the same
time, the forms of the patterns through which it has been transmitted.
The instrument serves also to illustrate most of the important phenomena
of colour.
XIII. _Simple Microscopes and Telescopes._
The Stanhope lenses are now sold at such a cheap rate, and are so useful
as simple portable microscopes, that it is hardly worth while to detail
any plan by which a cheap single-lens magnifier may be obtained.
Eloquent vendors of cheap microscopes are to be found in the streets,
who make their instrument of a pill-box perforated with a pin-hole, in
which a globule of glass fixed with Canada balsam is placed; and the
spherical form of the drop affords the magnifying power: or a thin
platinum wire may be bent into a small circular loop, and into this may
be placed a splinter of flint-glass; if the flame of a spirit-lamp is
urged upon the loop of platinum wire and glass by the blowpipe until it
melts, a small double-convex lens may be obtained, which will answer
very well as a magnifying-glass. Practice makes perfect, and after two
or three trials, a good single lens may be obtained, which can be
mounted between two small pieces of lead, brass, or cardboard, properly
fixed together, with holes through them just large enough to retain the
edge of the tiny lens. A prism can be made of two small pieces of
window-glass stuck together with a lump of soft beeswax, and if a few
drops of water are placed in the angle, they are retained by capillary
attraction. The prism is used by holding it against a large pin-hole or
small slit in a bit of card, and directing them towards the sky, when
the beautiful colours of the spectrum will be apparent if the card and
prism are brought close to the eye.
The most simple form of the refracting telescope is made with a lens of
any focal length exceeding six inches, placed at one end of a tin or
cardboard tube, which must be six inches longer than the focal length of
the lens; the tube may be in two parts, sliding one within the other,
and when the eye is placed at the other end, an inverted image of the
object looked at, is apparent. By using two double-convex lenses, a more
perfect simple astronomical telescope is obtained. The object-glass,
_i.e._, the lens next the object looked at, must be placed at the end of
a tin or pasteboard tube larger than its focus, and the second lens,
called the eye-glass, because next the eye, is a smaller tube, termed
the eye-tube; and if the focal length of the object-glass is three feet,
the eye-glass must have a one-inch focus, and of course the eye-tube and
glass must slide freely in the tube containing the object-glass. An
object-glass of forty feet focus will admit of an eye-glass of only a
four-inch focus, and will, therefore, magnify one hundred and twenty
times. A tube of forty feet in length would of course be very
troublesome to manage, and therefore it is usual to adopt the plan
originally devised by Huygens, viz., that of placing the object-glass in
a short tube on the [Page 320] top of a high pole with a
ball-and-socket joint, whilst the eye-glass is brought into the same
line as the object-glass, and focused with a tube and rack-work properly
supported. In an ordinary terrestrial telescope there are four lenses,
in order that the objects seen by its assistance shall not be inverted;
and whenever objects are examined by a common telescope, they are found
to be fringed, or surrounded with prismatic colours. This disagreeable
effect is corrected by the use of _achromatic_ lenses, in which two
kinds of glass are united; and the light decomposed by one glass,
uniting with the colours produced by the other form white light, thus a
double convex lens of crown glass, C C, may be united with a
plano-convex lens of flint glass, F F, which must have a focus about
double the length of that of the crown-glass lens. The concave lens
corrects the colour or chromatic aberration of the other, and leaves
about one-half of the refracting power of the convex lens as the
effective magnifying power of the compound lens. The French opticians
cement the lenses very neatly together, and use them in ordinary spy and
opera glasses. (Fig. 307.)
[Illustration: Fig. 307. A compound achromatic lens, composed of C C,
the double-convex lens of crown-glass, and F F, the plano-concave lens
of flint-glass.]
XIV. _The Stereoscope._
This instrument has now attained a popularity quite equal to, if it does
not surpass, that formerly enjoyed by the kaleidoscope, and without
entering upon the much-vexed question of priority of discovery, it is
sufficient again to mention with the highest respect the names of Sir
David Brewster and Professor Wheatstone as identified with the discovery
and use of this most pleasing optical instrument.
The principle of the stereoscope (meaning, _solid I see_) is copied from
nature: _i.e._, when both eyes are employed in the examination of an
object, two separate pictures, embracing dissimilar forms, are impressed
upon the retinæ, and produce the effect of solidity; if the pictures
formed at the back of the eyes could be examined by another person with
a stereoscope, they would come together, and also produce the effect of
solidity.
Stereoscopic pictures are obtained by exposing sensitized paper in the
camera to the picture of an object taken in two positions, or two
cameras are employed to obtain the same result. If the latter mode is
adopted, the stereoscopic pictures must not be taken from positions too
widely separated from each other; or else, when the two pictures are
placed in the stereoscope, they will stand out with a relief that is
quite unnatural, and the object will appear like a very reduced solid
model, instead of having the natural appearance presented by pictures
which have been taken at positions too distant from each other.
Sir David Brewster says, "In order to obtain photographic pictures
mathematically exact, we must construct a binocular camera which will
[Page 321] take the pictures simultaneously, and of the same size; that
is, by a camera with two lenses of the same aperture and focal length,
placed at the same distance as the two eyes. As it is impossible to
grind and polish two lenses, whether single or achromatic, of exactly
the same focal lengths, even if we had the very same glass for each, I
propose to bisect the lenses, and construct the instrument with
semi-lenses, which will give us pictures of precisely the same size and
definition. These lenses should be placed with their diameters of
bisection parallel to one another, and at a distance of 2½ inches,
_which is the average distance of the eyes in man_; and when fixed in a
box of sufficient size, will form a binocular camera, which will give us
at the same instant, with the same lights and shadows, and of the same
size, such dissimilar pictures of statues, buildings, landscapes, and
living objects, as will reproduce them in relief in the stereoscope."
Thus with a single camera provided with semi-lenses, or two lenses of
the same focal length, stereoscopic pictures can be obtained.
To bring the images of the two pictures together, and produce the effect
of solidity; either of two instruments may be employed. The reflecting
stereoscope is the invention of Professor Wheatstone. The refracting or
lenticular stereoscope that of Sir David Brewster.
The former is constructed by placing two upright boards on a wooden
stand at a moderate distance from each other; the stereoscopic pictures
are attached to these boards, which may be made to move up or down, and
if the pictures are held in grooves, they may be pulled right or left at
pleasure, and thus four movements are secured--viz., upward, downward,
right, or left. Between the two stereoscopic pictures are placed two
looking-glasses, so adjusted that their backs form an angle of ninety
degrees with each other. (Fig. 308.)
[Illustration: Fig. 308. Wheatstone's reflecting stereoscope.]
The pictures are illuminated at night by a lamp or gas flame placed at
the back of the mirrors, which, when fixed together, have the same shape
as a prism; indeed, Professor Wheatstone substituted a prism for the
mirrors, and thus paved the way for the invention of the lenticular
stereoscope.
[Page 322]
The stereoscopic effect is obtained by bringing the eyes close to the
inclined mirrors, so that the two reflected images coincide at the
intersection of the optic axis; the coincidence of the images is further
secured by moving either picture a little to the right or left, and if
the upright boards move bodily in grooves to or from the centre mirror,
the greatest nicety of adjustment is procured.
During the last three years of the author's directorship of the
Polytechnic--viz., in 1856, 1857, 1858--nearly the whole of the pictures
shown by the dissolving-view apparatus were coloured photographs from
Mr. Hine's original pictures, painted two feet square in blue and white,
and reduced on the glass to about six inches square. The collodion film
being frequently thick and difficult to penetrate with light, was etched
and scratched away where required, and filled in with colour, and when
these pictures were looked at with _one_ eye only, they appeared to be
almost solid or stereoscopic on the disc.
The lenticular stereoscope consists of a box of a pyramidal shape, open
at the base, and provided with grooves in which are placed the
stereoscopic pictures; if the latter are taken on glass the base of the
box is held directly against the light, but if they are daguerreotypes
or paper pictures, then a side light is reflected upon them by means of
a lid covered in the inside with tinfoil, which is raised or lowered at
pleasure from the top part of the box. Two semi-lenses are now fitted
into the narrow part of the box, and are placed at such a distance from
each other that the centres of the semi-lenses correspond with the pupil
of the eyes, and this distance has already been stated to amount to
2½ inches. (Fig. 309.)
[Illustration: Fig. 309. Brewster's lenticular stereoscope.]
The principle of the lenticular stereoscope is perhaps better seen by
reference to the next diagram, in which the centres of the semi-lenses
(_i.e._, a lens cut in half) are placed at 2½ inches apart, with
their _thin_ edges towards each other, and marked, A B, Fig. 310. The
centres of the two stereoscopic pictures C D correspond with the centres
of the lenses, and the rays of light _diverging_ from C D fall upon the
semi-lenses, and being refracted nearly _parallel_ are, by the prismatic
form of the semi-lenses, deflected from their course, and leave the
surfaces of the lenses in the same direction as if they actually
emanated from E; and as all images of bodies appear to come in a
straight line from the point whence they are seen, the two pictures are
superimposed on each other, and together produce the appearance of
solidity, so that a stereoscopic result is obtained when the _spectral
images_ of the two stereoscopic pictures are made to overlap each other.
By taking one of the semi-lenses in each hand, and looking at the two
pictures, the over-lapping [Page 323] of the _spectral images_ becomes very
apparent, so that the combined _spectral images_, and not the _pictures_
themselves, are seen when we look into a stereoscope. (Fig. 310.)
[Illustration: Fig. 310.]
Sir David Brewster says, "In order that the two images may coalesce
without any effort or strain on the part of the eye, it is necessary
that the distance of the similar parts of the two drawings be equal to
twice the separation produced by the prism. For this purpose measure the
distance at which the semi-lenses give the most distinct view of the
stereoscopic pictures, and having ascertained by using one eye the
amount of the refraction produced at that distance, or the quantity by
which the image of one of the pictures is displaced, place the
stereoscopic pictures at a distance equal to twice that quantity--that
is, place the pictures so that the average distance of similar parts in
each is equal to twice that quantity. If this is not correctly done, the
eye of the observer will correct the error by making the images
coalesce, without being sensible that it is making any such effort. When
the dissimilar stereoscopic pictures are thus united, the solid will
appear standing as it were in relief between the two plane
representations."
XV. _The Stereomonoscope._
M. Claudet, whose name has long been celebrated in connexion with the
art of photography, has described an instrument by which a single
picture is made to simulate the appearance of solidity, and he states
that by means of this arrangement a number of persons may observe the
effect at the same time. The apparatus required is very simple,
consisting of a large double convex lens, and a screen of ground glass.
The [Page 324] object A, Fig. 311, is highly illuminated, and placed in
the focus of a double convex lens B, when an image of the object is
projected, and will be found suspended in the air in the conjugate focus
of the lens at C, and from this point the rays of light will diverge as
from a real object, which will be seen by separate spectators at D D and
E E; and if the screen of ground glass is placed at G G, the image will
appear with all the effect of length, breadth, and depth, which belong
to solid bodies. (Fig. 311.)
[Illustration: Fig. 311. The stereomonoscope.]
An image formed on ground glass in this manner can be seen only in the
direction of the incident rays, and the stereoscopic effect is not
apparent when the image is received on a calico or transparent screen,
on account of the rays being scattered in all directions.
XVI. _The Stereomoscope._
[Illustration: Fig. 312. The stereomoscope.]
This arrangement is an important modification of the other, and consists
of a screen of ground glass (A B, Fig. 312), and two convex [Page 325]
lenses (C D, and E F) arranged in such a manner that they will project
images of the stereoscopic pictures, G H, at the same point on the
screen, A B.
It might be thought that a confusion of images would result from
projecting two pictures on one point, P--viz., the focus of the two
lenses; but as each photograph can be seen only in the direction of its
own rays, it follows that if the eyes are so placed that each receives
the impression of one stereoscopic picture, the two images must
coalesce, and a stereoscopic effect will be the result, as is apparent
at K K and L L; so that several persons may look at the stereoscope at
one time. (Fig. 312.)
XVII. _The Pseudoscope._
[Illustration: Fig. 313. Horizontal section of the pseudoscope, showing
at A B two prisms placed against a block of wood about two inches long
and one inch and a half wide, and cut out in the centre to admit the
nose at D. The eyes are supposed to be looking at the globe, C, in the
direction of the arrows. E E. Brass plates blackened, which shut out the
side light, and assist in keeping the prisms in position.]
This curious optical instrument, as its name implies, produces a false
image by the refracting power of prisms, and is the invention of
Professor Wheatstone. When used with both eyes, the same as the
stereoscope, it inverts the relief of a solid body, and makes it appear
exactly as if it were an intaglio, or sunk beneath the line surrounding
it. For instance, a terrestrial globe when looked at through the
pseudoscope appears to be concave, like Wyld's Globe in
Leicester-square, instead of convex. A vase with raised ornaments upon
it looks as if it had been turned (to reverse the usual expression)
outside in, and [Page 326] the whole of its convexity is turned to
concavity; and of course a face seen under these circumstances looks
very curious. (Fig. 313.) The cause is perhaps somewhat difficult to
understand; but by taking other and more simple examples of the same
effect, the principle may be gradually comprehended.
Sir David Brewster, in his "Letters on Natural Magic," remarks that "one
of the most curious phenomena is that _false_ perception in vision by
which we conceive depressions to be elevations, and elevations
depressions--or by which intaglios are converted into cameos, and cameos
into intaglios. This curious fact seems to have been observed at one of
the early meetings of the Royal Society of London, when one of the
members, in looking at a guinea through a compound microscope of new
construction, was surprised to see the head upon the coin depressed,
while other members could only see it embossed, as it really was.... The
best method of observing this deception is to view the engraved seal of
a watch with the eye-piece of an achromatic telescope, or with a
compound microscope, or any combination of lenses which inverts the
objects that are viewed through it; a single convex lens will answer the
purpose, provided we hold the eye six or eight inches behind the image
of the seal formed in its conjugate focus."
After bringing forward various interesting experiments in further
explanation of the cause, Sir D. Brewster states it to be his belief
that the illusion is the result of an operation of our own minds,
whereby we judge of the forms of bodies by the knowledge we have
acquired of light and shadow. Hence, the illusion depends on the
accuracy and extent of our knowledge on this subject; and while some
persons are under its influence, others are entirely insensible to it.
This statement is borne out by experience, as the author, whilst
Resident Director of the Polytechnic, had four of Wheatstone's
pseudoscopes placed in the gallery, with proper objects behind them; and
he frequently noticed that some visitors would look through the
instrument and see no alteration of the convex objects, whilst others
would shout with delight, and call their friends to witness the strange
metamorphosis, who in their turn might disappoint the caller by being
perfectly insensible to its strange effects.
The pseudo-effects of vision are not confined to the results already
explained, but are to be observed especially whilst travelling in a
coach, when the eyes may be so fixed as to give the impression of
movement to the trees and houses, whilst the coach appears to stand
still. In railway carriages, after riding for some time and then coming
to a stand still, if another train is set slowly in motion by the one at
rest, it frequently happens that the latter appears to be moving instead
of the former.
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