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24 The Optical Magic Lantern Journal and Photographic Enlarger.
The construction of stereotropes, or instruments for giving apparent motion to the objects pictured on ordinary stereoscopic slides, is not new, but the subject has died out of public memory for almost thirty years, and the interest in its revival at the present moment consists in the raising of the question whether inventors will hereafter succeed in making the principle available for use with the magic lantern. A little is therefore appended about some of these curious old instruments, a description of which will probably come as a revelation to some lanternists, who are members of the rising generation.
In the supplement to The Philosophical Magazine for 1861, vol. 22, fourth series, page 537, is a description of a stereotrope exhibited at a meeting of the Royal Society, January 10th, 1861, by Mr. W. J. Shaw. The inventor took eight stereoscopic pictures of an object, say a steam engine, the pictures representing the successive positions it takes in completing one revolution of the fly-wheel, and mounted them upon an octagonal drum, which revolved upon a horizontal axis; the stereograms were then viewed through an ordinary lenticular stereo3cope as they came one by one into view. Immediately beneath the lenses, and with its axis situated half an inch from the plane of sight, was fixed a’ solid cylinder, four inches in diameter, capable of being moved freely on itsaxis. This eye-cylinder was pierced through its entire length by two aperatures, so that a transverse section of the cylinder shows them as cones, with their apices pointing in opposite directions, and with their axes parallel to, and distant half an inch from the diameter of the cylinder. In the middle of each of these conical holes was a diaphragm. The eye-cylinder and the picture drum were so geared together, that the former made four revolutions to each single revolution of the latter. The effect of this was, that only eight times in a revolution of the drum could a stereogram be seen, and then, by means of persistence of vision, the machine in the picture seemed to be at work. Mention is madeat theend of thearticle of another stereotrope, not described, in which use is made of the reflecting stereoscope. There is nothing about Mr.Shaw’s paper in Philosophical Transactions for 1861, but something about it in The Proceedings of the Royal Society for that year, from which it appears that Mr. Shaw's memoir was communicated to the Royal Scciety by Mr. Warren De La Rue, and the remark is made, that in stereotropes, Wheatstone’s reflecting stereoscope is ‘‘ better adapted for the exhibition of movements that are not only local but
progressive in space.” There are no diagrams of the instrument, and to give clear ideas to the readers, written descriptions of mechanical appliances should always be accompanied by simple drawings to explain the principles of construction.
The era of stereotropes was a short one, which lasted only for a few years before and after 1860. Duboscq, the celebrated optician of Paris, invented one of them, but we have not yet been able to find a full description of his instrument. His papers on optical instrumenis were sometimes accepted by the Academy of Sciences, but a description of his stereotrope is not among the number. In 1859, a German book on the stereoscope was published in Quedlinberg, and included some new stereoscopic instrument by Duboscq, but we have not yet been able to see a copy of the work, so that all about it at present accessible to us is contained in a paper read in 1865 before the British Association in Birmingham, by Mr. Claudet, in which the latter says of stereotrope inventors : ‘‘ He (Duboscq) had fixed two series of binocular photographs upon two zones of the revolving disc of the phenakitiscope, one above the other, and by means of two small mirrors, placed each respectively at the inclination capable of reflecting the two zones on the same horizontal line, whence the images could each separately meet the axes of each of the two prismatic lenses of the stereoscope, each eye, during the revolution of the disc, had separately the perception of one of the series of photographs, each showing the perspective of one eye, and the stereoscopic effect of figures in motion was consequent.” Claudet goes on to state that M. Ducoscq gave likewise another form to the instrument. Instead of the vertical revolving disc, he used a cylinder revolving on its vertical axis, and he placed on two inside zones of that cylinder, one above the other, the two series of photographic pictures, and two slits through which the eyes could see the pictures; and by means of two mirrors, as in the other apparatus, each series was reflected on its respective lens through the cylinder, thus giving the effect of motion to the object represented in the stereogram.
Claudet stated that the defect of one of these arrangements was, the two series of pictures did not move with the same velocity, as they moved in two circles of different diameter, thus introducing a certain amount of distortion. With the revolving cylinder the defect did not occur, but the pictures were considerably curved, like the cylinder, which was unfavourable to their inspection in the stereoscope.