Discoveries and Inventions of the Nineteenth Century

of the modelling of the mountain could be thus obtained than even a spectator of the actual scene would receive by viewing the mountain from any one point. Such a mode of combining the photographs would produce in the stereoscope the same effect as the telestereoscope would in the landscape, but the effect would be caused to a proportionately far higher degree. The date of Wheatstone’s first publication regarding the stereoscope was 1833; but a complete description and theory of the instrument was not published until five years afterwards. Brewster first made public, in 1843, his invention of the stereoscope with lenses, which is now so familiar to us, and few scientific instruments have become so quickly and extensively popular; certainly no other simple and inexpensive instrument has contributed so largely to the amusement and instruction of our domestic circles. And, to the philosopher who studies the nature of our perceptions, the stereoscope has been even more instructive, for, instead of vague surmises, 475it provided him with the solid ground of experiment on which to found his theories. The literature of this one subject—stereoscopic effect—is extensive enough to occupy a tolerably long book-shelf. It dates from 300 B.C., when Euclid touched upon the subject in his Optics; and after a lapse of more than eighteen centuries it was taken up by Baptista Porta, in 1583; but the whole development of this subject belongs almost entirely to the last half-century. Fig. 247. The part which the muscles of the eyes take in our perceptions of form has been already alluded to, and it may be interesting to illustrate this point by a curious example or two of illusions arising from their movements. If our reader will glance at Fig. 247, he will see that the lines, a b and c d, appear to be farther apart towards the centre than at the ends, while f g and h i, on the other hand, appear nearest together in the middle. He will hardly be convinced that in each case the lines are quite parallel until he has actually measured the distances. A still more striking example of the same kind of illusion is shown by Fig. 248, due to Zöllner. This appears a sort of pattern, in which the broad bands are not upright, but sloping alternately to the right and left, and with the spaces between the lines wider at one end than the other. The lines in the figure are, however, strictly parallel. The illusion by which they appear divergent and convergent is still more strongly felt when the book is held so that the wider bands are inclined at an angle of 45° to the horizon. There is another illusion here with reference to the short lines, which will appear to be opposite to the white spaces on the other side of the long lines to which they are attached. That these illusions are really due to movements of the eyes may be proved by viewing the designs in any manner which entirely prevents the movement, as by fixing the gaze on one spot in the case of Fig. 247, when the illusion will vanish; but this plan is not so easily applied to Fig. 248. A convincing proof, however, will be found in the appearance of these figures when they are viewed by the instantaneous light of the electric spark, as when a Leyden jar is discharged in a dark room. The reader viewing the figures, held near the place where the spark appears, 476will see them distinctly without the illusions as to the non-parallelism of the lines. In the absence of an electrical machine, or coil and jar, the reader may have an opportunity of seeing the figures by flashes of lightning at night, when the result will be the same. Fig. 248. There is a property of the eye which has led to the production of many amusing and curious illusions. This property in itself is no new discovery, for its presence and effects must have been noticed ages ago. The property in question is illustrated when we twirl round a stick or cord, burning with a red glow at the end. We seem to trace a circle of fire; but as the glowing spark cannot be in more than one point of the circle at once, it is plain that the impression produced on the eye must remain until the spark has completed its journey round the circle, and reaching each point successively renews the luminous impression. Like other subjects relating to vision, this phenomenon has been carefully examined in recent times, and its laws accurately determined. The fact which is obvious from such an experiment, may be thus stated: Visual impressions repeated with sufficient rapidity produce the effect of objects continually present. This persistence of the visual impressions is easily made the subject of experiment by means of rapidly rotating discs; and in the common toy called a “colour top” we have a ready means of verifying some of the conclusions of science on this subject. Some very interesting results may be obtained by an apparatus as simple as this, regarding the laws of the phenomenon we are considering, and the effects of various mixtures of tints and colours. The well-known toy, the thaumatrope, depends on the same principle. In this a piece of cardboard is painted on one side, with a bird, for example, and on the other side with a cage: when the cardboard is twirled round very rapidly by means of a cord fixed at opposite points of its length, both bird and cage become visible at once, and the bird appears in the cage. Fig. 249. Fig. 250. Fig. 251. A still more ingenious application of this principle we owe to Plateau, who described it in 1833, under the title of the phenakistiscope; and also to Stampfer, who independently devised the same arrangement about the same time, and named it the stroboscopic disc. The reader may, at almost any toy-shop, purchase one of them, provided with a number of amusing figures; or he may easily construct for himself one which will exemplify the principle. He requires no other materials than 477a piece of cardboard, and his only tools may be a sharp penknife, a pair of compasses, and a flat ruler. Let him draw on his cardboard a circle of 8 in. diameter, and divide its circumference by eight equidistant points. From these radii should be drawn with the point of the compasses, and equal distances from the centre marked off upon them, to fix the centres of the small circles, which must all have exactly the same size (say, 1 in. in diameter) and be marked by a distinct line. In these are to be marked the hand of a clock-face in the positions shown in Fig. 249; and finally, in the direction of the radii, narrow slips are to be cut out of the cardboard as shown. If a pin be put through the centre of the disc, attaching it thus to the flat end of a cork, so that it can freely rotate in its own plane, and the disc be turned rapidly round, as in Fig. 250, in front of a looking-glass, while the spectator looks through the slits, he will see the hand on the little dial apparently turning round, with rather a jerky movement it is true, somewhat like the dead-beat seconds-hand that is sometimes seen on clocks. The illusion is best when the slits are so narrow that only one of the several images is visible by reflection, namely, that which is adjacent to the slit. Thus, as the disc rotates, each little circle is visible for an instant as the slit passes in front of the spectator’s eye; and if the rotation be sufficiently rapid, the impression of the disc is permanent, as it is constantly being renewed by the successive circles, while, on the contrary, the hands, having different positions, produce images in different positions, giving the appearance of a jerky rotation. The instruments sold in the shops have sometimes a thin metallic disc with the slits in it, and a series of designs printed in smaller paper discs. The paper discs may be screwed on the other disc as required, and a button on a pulley with an endless band is provided for producing the 478rotation more conveniently. Fig. 251 shows one of the pictures for a disc with twelve slits, and the effect produced by it is that of a dancing figure. Another arrangement for showing the same illusion has lately become a very popular toy, and quite deservedly so, for it has the advantages of requiring no looking-glass, and of making the effect visible to a number of persons at the same time. This apparatus, which has been termed the Zoetrope, consists simply of a cylindrical box, like a drum with the upper end cut off. It is mounted on a pivot, which permits its revolving rapidly about its vertical axis when touched by the finger. The cylinder has a number of equidistant vertical slits round the upper part of its circumference. The figures which produce the illusion are printed on a slip of paper, which is placed in the lower part of the drum, and when this is in rapid rotation, and the figures are viewed through the slits, the illusion is produced in exactly the same manner as in the revolving disc. Fig. 251a.—Edison’s Kinetographic Theatre. At the end of the article on the phonograph in a subsequent page, the reader will find a remark as to the effect that might be produced by a combination of that instrument with instantaneous and simultaneous photographs of some famous speaker. This combination has now been accomplished by the great inventive genius to whom we are indebted for the phonograph. Mr. Edison has done this so effectively that he may be said to have given life to the zoetrope by the perfection in which the ocular illusion is produced together with the audible manifestations that keep time with it. The amount of thought and ingenuity expended on this new contrivance, which Edison has called the kinetoscope, will scarcely be appreciated by anyone who has not given some consideration to the many practical difficulties that have been overcome. No wonder that the announcement made at the beginning of 1892 should have been received with incredulity, for it was to the effect that Edison had contrived some happy combination of photography and electricity by which a man (presumably one who could afford to pay for luxuries) might sit in his own room and see the moving forms of the actors in an opera projected on a screen before his eyes, while at the same time he would hear their voices singing. Every movement, every change of expression, every glance of the eye, and, in fact, all that was visible to the spectator in front of the stage would appear on the screen, while not a note of vocalist, or chord of orchestra, would fail to reach the ear. And all this was to be evoked at will, and repeated as often as desired, not, therefore, of course, as a presentation of what was taking place at the time, but as a reproduction of some previous performance. This wonderful result has virtually been attained by the application of delicate and ingenious machinery designed to make the phonograph and the camera work synchronously. The first part of the problem was the production of a succession of so-called instantaneous photographs at an extremely rapid rate. In the actual apparatus forty-six photographs are taken every second, a feat which would beforehand be thought impracticable. This is accomplished by making use of a band of sensitive celluloid film, which alone admits of being moved and stopped with the desired rapidity. The movement is imparted by an electric motor, and the arrangement is such that for each exposure the film is held stationary for 910ths of 146th of a second, during which the lens is uncovered, then for the remaining ⅒th it is covered, while at the same time the film