The Optical Magic Lantern Journal (February 1896)

24 The Optical Magic Lantern Journal and Photographic Enlarger. screen may be seen by an observer unfurnished with any special observing device. The two images forming the stereogram were superimposed upon the same part of a ground glass screen, and in the course of one set of rays was interposed a blue glass, and in the course of the other set of rays a yellow glass. Assuming the images to be very small, and the axial ray of each projection system to be suitably inclined in a horizontal plane, it is easy to imagine a position on the other side of the ground glass screen, where one eye of the observer would receive chiefly yellow rays, and the other eye would chiefly receive blue rays, so that the image would be seen with some stereoscopic effect—an effect which would doubtless be much confused by the irregular reflection from the ground glass surface. It must be remembered that at this period the now almost forgotten researches of Dove (1841) had made the scientific world aware that if one stereoscopic element of a solid figure is outlined in blue on a white ground, and the other stereoscopic element of the figure is outlined in red on the same sheet, a spectator furnished with a blue glass over one eye, and a red glass over the other eye, will see the stereoscopic union of the two coloured images. Dove's results in this direction are referred to and discussed by Tyndall in a paper which will be found in Humphrey’s Journal of Photography, volume for 1852-53, issue of March 1, 1853, The above device of Dove is substantially a foreshadowing of the recent anaglyph of Ducos du Hauron, and it may be assumed that De la | Blanchére’s device was an effort to improve it by eliminating the discriminative coloured | spectacles. In 1858 J. Ch. D’Almeida, in a communication to the French Academy of Sciences, set forth very clearly the method of stereoscopic projection by distinctive colours and discriminating spectacles; but he does not appear to have known of the earlier work in the same direction. The original communication of D’Almeida will be found in the 47th volume of the Comptes fendus de l’Acadamie des Sciences, at page 61, this volume being the second volume for 1858. A nearly full translation is to be found in the | issue of the Journal of the Photographic Society for August 21st, 1858, page 2, and all the conditions of working are so clearly put that a short quotation may be given:—‘ For this purpose the author places in the course of the luminous rays two coloured glasses which have no element, or scarcely any element of the spectrum in common. One of these is the red glass, well known | to physicists; the other the green glass, which he obtained in the shops. By means of these coloured glasses, one of the images thrown upon the screen is rendered green and the other red; and similar glasses being placed before the eyes, the green image alone will be perceived by the eye covered by the red glass, and the other to that furnished with the red glass. The relief then appears immediately. A very remarkable modification is observed on moving to one side; it appears then as if one saw all the changes which would be_ perceived if the objects were really in relief.” D’Almeida then describes another method— that of projecting the two images alternately by means of a revolving disc placed in the path of the light of the two projecting lanterns. As regards the observer, he says: ‘‘ Whilst this wheel turns, the eyes look through another cardboard revo'ving on the same axis, and pierced with holes in the same way. The right eye can only see at the moment when the right perspective appears, and the left eye also becomes free when the left perspective makes its appearance ; this may be effected by a small electro-magnetic apparatus.” (To be continued.) —:0:—— Making a Dissolving Tap. By Rev. Wu.>H. Youne, Ph.D. EVERYONE appreciates the finest apparatus, but too often it must be through shop windows only. When compelled to do without such luxuries, we must be content with what we can make, and it is surprising what accurate and effective apparatus can be made out of homely materials. Indeed, the most celebrated artisans and scientists seem to prefer home-made implements, as was indelibly impressed upon the writer while he was privileged to assist the late Professor Joseph Henry in the Smithsonian Institution. Those lanternists who have more skill than money, may easily make a perfect six-way dissolving tap for about five shillings. First get two new double bracket cocks made for gas fixtures, Fig. J. (a). See that they are air-tight by blowing or sucking. These are then to be mounted in any desired way on a piece of wood, as in Fig. II. ; one of them connecting with the oxygen bag or bottle, the other with the hydrogen or saturator.