The Boy's Playbook of Science Part 37

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 gla.s.s, for the purpose of focussing the picture which is inverted and placed between the two lenses. (Fig. 293.)

[Ill.u.s.tration: Fig. 293. The magic lantern.]

In many books full directions are given for painting the gla.s.s 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 gla.s.s is concerned, and unless the juvenile artist has taken lessons from a regular painter on gla.s.s, 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 ill.u.s.tration, in the preparation of photographs on gla.s.s 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 a.s.sistance 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.)

[Ill.u.s.tration: 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 bra.s.s 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.)

[Ill.u.s.tration: 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 gla.s.ses 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.)

[Ill.u.s.tration: 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 ill.u.s.trating 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 sh.e.l.ls, &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 bra.s.s worked by the hand, and the lower ones with the fans. (Fig. 297.)

[Ill.u.s.tration: 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 a.s.sisted by various sounds in imitation of war's alarms, for the production of which, more _volunteers_ than were required would occasionally trespa.s.s 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 gla.s.s 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.

[Ill.u.s.tration: 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 gla.s.s. The objects, such as live aquatic insects, are placed in gla.s.s troughs containing water; the other objects, ferns, feathers, b.u.t.terflies, algae, &c. &c., being mounted on slides in the ordinary way with Canada balsam. (Fig. 298.)

[Ill.u.s.tration: 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 gla.s.ses.]

IV. _The Physioscope._

This instrument, brought out at the Polytechnic during the time that Mr.

J. F. G.o.ddard managed the optical department of the inst.i.tution, always excited the greatest mirth and astonishment amongst the numerous visitors; and _habitues_ 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 gla.s.s 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]

[Ill.u.s.tration: 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 gla.s.ses. 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.)

[Ill.u.s.tration: 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 a.n.a.lysis and Synthesis."_

It is in the Italian language that the bride, the emblem of purity, is called Lucia (_Lux_, light); and surely if an ill.u.s.tration 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.)

[Ill.u.s.tration: 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 a.n.a.lysis of light, by receiving a sunbeam (as it pa.s.sed 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 pa.s.sing 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.)

[Ill.u.s.tration: Fig. 302. A. The ray of light pa.s.sing through two prisms B placed base to base. In this position the light pa.s.ses 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 ill.u.s.trated 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.)

[Ill.u.s.tration: Fig. 303. No. 1. A. The electric light. B. The narrow slit through which the light pa.s.ses 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.]