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Stereoscopic Projection 857

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STEREOSCOPIC PROJECTION 857. Classification of the Various Methods. The various known methods of stereoscopic projection may be classified as follows— (A) The images are projected one on the other and are selected— (I) By alternately cutting out the two pro jected beams, with synchronized cut-out of the individual viewing apparatus.

(2) By their colour, each observer being supplied with an appropriate hi-coloured pair of spectacles.

(3) By polarization in different directions of the light in each of the projecting beams, each observer being provided with an analysing apparatus.

(4) By purely geometric means of less general application, stereoscopic vision only being pos sible to a single observer, or several observers in certain definite positions.

(B) The images are projected side by side, the enlarged stereoscopic pair on the screen being examined (i) By a mirror stereoscope.

(2) By special binoculars.

(3) By prisms.

(4) By a mask permitting each eye to see only one image.

858. Alternating Viewing. This method, some times called strobo-stereoscopy, was described in 1858 by J. Ch. d'Almeida. The two images are thrown by two projectors illuminated alter nately with a frequency of about 15 exposures per second for each image. In front of each observer a system of two eye-holes or two eye pieces is provided with shutters synchronized with those of the projectors, so that the right eye sees only the right image and the left eye the left image, with persistence of vision for both eyes.

Nothing appears to show that these projectors were actually made by d'Almeida. The appli cation of this principle to stereoscopic cinemato graphy (with the possibility of using only a single projector, the film consisting of alternate right and left-hand pictures of the same scene) has been patented many times since the begin ning of cinematography, notably by Dr. Doyen (Iwo), by Grivolas (1901), by E. Reynaud (1902), and by C. Schmidt and C. Dupuis (1903) ; the latter ran a cine-stereoscopic show for several months in Rue d'Hauteville, Paris ; each spec tator was provided with a species of opera glass attached to his chair by electric wires, which controlled an electro-magnetic shutter.

859. Projection in Two Complementary Colours. In the same paper in which the above process was suggested, d'Almeida wrote of a. method of stereoscopic projection, using pro jecting beams of red and green for the two images of the pair. By placing in front of the eyes red and green glasses complementary to each other, the red beam is extinguished by the green filter so that the eye, seeing through the green filter sees only the image projected with green light, whilst, for the same reason, the other eve, looking through the red filter, sees only the image projected with red light.

Soon after this publication, Rollinann claimed the priority of a similar process for the projec tion of stereoscopic pictures (§ 833).

Many attempts have been made to reduce the fatigue resulting from the very different sensa tions experienced by the two eyes by using, both on the projectors and on the analysing glasses, complementary filters of nearly neutral tints. F. E. Ives in 1896 claimed to have succeeded in making two filters practically uncoloured, one transmitting the extreme red and a narrow band in the middle of the green, whilst the other transmitted two other spectral groups, one, on both sides of the yellow, and the other in the blue-violet region. The same results were obtained about 1906 by R. Luther.

A paper was published by H. Lehmann (1917), describing an apparatus for stereoscopic pro jection of ordinary stereograms in comple mentary colours (by ordinary stereogram is meant two black images side by side). A lantern using an arc and single condenser is fitted with two equal prisms joined by their edges along the line of junction of the two images of the pair, and two lenses of variable separation. He also points out that by suitable adjustment of the regions of transmission of the filters used, so that the transmitted luminosi ties are equal, it is possible to suppress all effect of colour in the projection of the pictures in black and white, for an observer seeing normally in colours, or to project stereograms in colour without alteration of their tints. He indicates the following liquid filters as fulfilling approxi mately the above conditions : a solution of copper sulphate saturated when cold in a cell 15 mm. thick, and a solution of potassium bichromate saturated in the cold and acidified with sulphuric acid in a cell 10 mm. thick.' This method of projection, employing two complementary colours, has been claimed, in its application to cinematography, by Grivolas, in a patent in 1901, and since then by innumerable inventors. The different variations of the pro cess employ either two simultaneous projectors or alternate projection of the left and right-hand pictures with the use of suitable colour filters on the shutter or alternate tinting of the images themselves on the film.

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