Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

in this direction were applied to the movements of a horse moving at full speed. The pictures, taken in series, showed that he performed muscular actions that were not before comprehended or even imagined. These pictures at the time attracted great attention, and instantaneous pictures have been since taken of dancers in a ball-room, of vessels and steam-boats in rapid movement, of all kinds of animals in motion, and of machinery in operation. As the pictures represent the movements at one instant of time, they give, as it were, a fixed view of a motion, precisely as if it were suddenly arrested in full action. In the case of animals, the motions of the nostrils are represented in the most singular manner, and the spokes of a steam-boat’s paddle-wheel are shown apparently perfectly still while the spray and waves appear in active motion, or, rather, as they would look if they could be instantly frozen. It is clear the new process and pictures will open a wide and instructive field in art and in the study of mechanical action.

      While on the subject of Photography we may mention a very ingenious little apparatus called a Scenograph, the invention of Dr. Candize. It is really a pocket-camera, and is so easily manipulated that it will be found a most pleasant and useful holiday companion. Any one may obtain good results with it, and friends of ours have had occasion to put it in practice during a series of excursions, when it was found to answer in every instance.

      The Scenograph is something like a common Stereoscope in outward appearance, and would, perhaps, be at first regarded as a mere toy, did not a more intimate acquaintance prove it a great acquisition, particularly to explorers and tourists. The tripod stand which supports the apparatus is, when not in actual use in that capacity, a very excellent walking-stick, in which the two other “legs” are carried. The instrument, as will be perceived from the illustration (fig. 166), is very handy. It produces pictures about the “cabinet” size, and the whole is so arranged that it can be packed and carried in the pocket with ease.

      Fig. 166.—The Scenograph.

      Photography, as a rule, necessitates a dark room or cabinet, and many preparations as we all know—a “messing” about with chemicals and considerable practice before we can become proficient; so it is not surprising that few amateurs take to it—they prefer to purchase the pictures. But in the new apparatus of which we are speaking, the glass plates are already prepared to receive the image. It is not at all necessary for the operator to stain his fingers and knuckles and nails with nitrate of silver, or any other “chemicals” whatever. He just inserts the plate in the Scenograph, and then his apparatus being steadily set up, he removes the covering from the lens. To develop the image (in the dusk of the evening or by candle-light) it is necessary to put some drops of ammonia in a saucer, breathe upon the plate so as to soften the collodion, and hold it above the ammonia, and then, under the influence of the vapour, the picture will appear. After this simple operation the picture will be found fixed for a lengthened period—practically indefinitely. Thus on the return of the pedestrian he can reproduce, at small expense, a whole series of little pictures faithfully representing his holiday tour. The illustration shows a small apparatus by which on thin plates small photographs can be taken and fixed till it is found desirable to enlarge them.

      The Photophone, one of the most recent contributions to science, is an instrument which, in combination with the telephone principle, makes it possible to convey sounds by means of a ray of light, and by means of a quivering beam “to produce articulate speech at a distance. The success of the Photophone depends upon a rare element, selenium, which has its “electrical resistance” affected by light. Professor Adams demonstrated that the resistance of selenium was reduced just in proportion as the intensity of the light which was acting upon it. Here was the key to the Photophone as thought out by Professor Bell. He fancied that he might by means of his telephone produce sound if he could vary the intensity of the beam of light upon the selenium, which he connected with his telephone and battery.

      The Photophone consists of a transmitter for receiving the voice and conveying it along the beam of light, and a receiver for taking the light and converting it into sound—the receiver being the telephone. There is a small mirror (silvered mica has been used) suspended freely for vibration. A lens is used to transmit to this the beam of light, and this beam is again reflected by another lens to the receiver, which consists of a reflector which has a cell of selenium in its focus, connected, as already stated, with the telephone and battery. The speaker stands behind the mirror, and the sound of his voice against the reverse side makes it vibrate in unison with the sounds uttered. The movements cause a quivering in the reflected beam, and this in its changing intensity acts on the selenium, which changes its resistance accordingly, and through the telephone gives forth a sound!

      This is the apparently complicated but really simple, and at the same time wonderful, invention of Professor Bell. By the Photophone not only sounds but movements can be converted into sound; even the burning of a candle can be heard! The Photophone is still capable of improvement, and has not as yet arrived at its full development, for it is stated it can be made quite independent of a battery or telephone.

      There are many phenomena connected with the Polarization of Light. This requires some notice at our hands. We know that a ray of ordinary light is supposed to be caused by vibrations of the highly attenuated medium, æther. These vibrations occur across the direction of the ray; but when they occur only in one plane the light is said to be “polarized.” Polarization means possessing poles (like a magnet); the polarized rays have “sides,” as Newton said, or, as explained by Dr. Whewell, “opposite properties in opposite directions, so exactly equal as to be capable of accurately neutralizing each other.” There are some crystals which possess the property of “double refraction,” and thus a ray of common light passing through such a crystal is divided into two polarized rays, taking different directions. One is refracted according to the usual laws of refraction; the other is not, and the planes of polarization are at right angles. It is difficult within the limits of this chapter to explain the whole theory of Polarization. In order to account for certain phenomena in optics, philosophers have assumed that rays possess polarity; and polarized light is light which has had the property of Polarization conferred upon it by reflection, refraction, or absorption. Common light has been compared to a round ruler, and polarized light to a flat ribbon. Huygens found out, when engaged upon the investigation of double refraction, that the rays of light, divided by passing through a crystal (a rhomb) of Iceland spar, possessed certain qualities. When he passed them through a second rhomb, he found that the brightness, relatively, of the rays depended upon the position of the second prism, and in some positions one ray disappeared entirely. The light had been reduced to vibrations in one plane. In 1808, Malus, happening to direct a double refracting prism to the windows then reflecting the sunset, found that as he turned the prism round, the ordinary image of the window nearly disappeared in two opposite positions; and in two other positions, at right angles, the “extraordinary” image nearly vanished. So he found that polarization was produced by reflection as well as by transmission. The differences between common and polarized light have been summed up by Mr. Goddard as follows:—

Common Light	Polarized Light
“Is capable of reflection at oblique angles of incidence in every position of the reflector.	“Is capable of reflection at oblique angles only in certain positions of the reflector.
“Will pass through a bundle of plates of glass in any position in which they may be placed.	“Will only pass through such glasses when they are in certain positions.
“Will pass through a plate of tourmaline, cut parallel to the axis of the crystal, in every position of the plate.”	“Will only pass in certain positions, and in others will not pass at all.”

      The bundle of glass plates or the tourmaline plate is thus the test for polarized light, and is termed an analyzer.

      The arrangement called a “Nichol’s prism,” made by cutting