The Optical Magic Lantern Journal (August 1896)

124 carrying out this idea, which in its present state is perhaps to be looked upon_ rather as an interesting, theoretical possibility than a matured method now available for every day practice. Having indicated every method of stereoscopic projection upon the screen with which I am familiar, I propose now to 1nake a number of discursive suggestions for improving and developing projection stereoscopy, or for so bringing it before the public, that a general and growing interest in the subject may lead to Improvements. In order to create interest in the matter, and to direct attention to the possibilities of stereoscopie projection, I would suggest that some enterprising manufacturer of lantern appliances might easily put up for sale in a cardboard box, at the price of say 2s. 6d., a pair of coloured glasses, and half-a-dozen pairs of bi-coloured spectacles, along with such instructions as would enable anyone having two lanterns (whether a pair or not), or a bi-unial, and some one or more stereoscopic transparencies, to introduce lantern stereoscopy as a feature of his entertainments; but of course half a-dozen pairs of spectacles would not serve for a large assembly ; still as coloured spectacies can now be had in the toy shops for a half-penny a pair, and those required for our present use need not cost more, extra spectacles could be easily provided as required. The question as to the colours of the glasses would have to be considered, and of all available combinations, I believe the copper ruby and signal green, adopted by Mr. Freshwater, to be one of the worst for general purposes. A rough test of the suitability of any two coloured glasses is their high transparency ; when not superimposed, and their high opacity when superimposed ; and any person who hasa. | large assortment of samples of coloured glass available for trial may easily satisfy himself that a gold pink and yellowish green will best satisfy these conditions. Owing, however, to the fact that gold pink glass is hardly a current commercial article, and when obtainable it is expensive and generally has that irregular surface which makers of church windows prefer, I recommend the next most efficient combination ; cobalt blue glass—not too dark, and a rather deep yellow inclining to orange. The colours must, however, be selected in reference to the criteria I have given— maximum of light when side by side, and maximum of darkness when superimposed, This is the combination originally employed by Blanchére in 1853, and not only are such glasses to be had in commerce The Optical Magic Lantern Journal and Photographic Enlarger. at alow rate and uniform in substance, but a first-class oil lamp is very nearly as efficient with the yellow glass as is an indifferent limelight ; hence excellent results can be obtained with a lime lantern for the blue, and an oil lantern for the yellow. Identity of lenses is not necessary, as the size of the images can be adjusted by a suitable distance of the lanterns. Should a demand arise for such bi-coloured slides, for use with a single lantern, as are mentioned above, they could be best printed directly on the glass, in coloured gelatinous inks, by the Woodbury method, but such slides have the following disadvantage over the BlanchereD’Almeida system, in which two lanterns are used, and the halves of an ordinary transparency are backed up by suitably coloured glasses. In the former case (t.¢c., the Du Hauron-Demole single slide) the white illumination of the screen is a real white, and corresponds to what the lantern will transmit through the clear glass of the slide. When the confused image is viewed through the coloured spectacles, each eye only sees certain selected coloured elements out of this white, and these coloured elements being approximately complimentary, the effect of a fictitious white is produced. Consequently there is an enormous falling off of the illumination when the coloured spectacles are used, and the impression of bad illumination is given. In.the two-lantern method the whites of the screen image are formed not of real white, but of the fictitious white resulting from the union of the complimentary tints, and on using the coloured spectacles, there is not the same kind of falling off in illumination as in the other case. I have, however, found it practicable to illuminate by fictitious white in the case of the one-lantern method, and so avoid that sudden fall of the illumination which is fatal to the succees of an exhibition. A screen is made of equal strips or squares of colours corresponding to those of the images forming the slides, and this is placed either close to the illuminant or in the place of the lens diaphragm as may be convenient, the advantage © of using strips or small squares being that the screen may be placed at some distance from one of the theoretical positions; indeed, quite in front of the objective in many cases. When stereoscopic subjects are to be shown in the course of a general exhibition the audience should be prepared for the weaker illumination by a short interval of comparative darkness, during which time a few words may’ be said as to the nature of stereoscopic vision, also as to the difference between such modelling ~ as can be painted on a flat surface and: true”