The Optical Magic Lantern Journal (September 1892)

The Optical Magic Lantern Journal and Photographic Enlarger. Tt may now be said to have parted with one half of itself. The methods adopted for polarising the light and making the polarised light available for the purpose required, are bricfly these: In front of each objective of an ordinary pair of lanterns a number of thin plates of glass are placed at the required angle, the light passes through thcse and the image of the slide is duly projected upon the screen. This picture, say from the top lantern, has nothing whatever to distinguish it from an ordinary one when seen by the unaided eye, but if it be looked at through a number of plates of thin glass held at an angle, it will be seen to possess peculiar properties, for in one position it will be clearly seen. If now the thin glass plates be turned through a quarter of a circle, the vicw nearly disappears, turned through another quarter it appears again. It will therefore be seen that the means are at disposal to either allow the picture on the screen to be seen, or to be, as it were, shut out, and also that whether it be visible or lost to sight entirely depends upon the position in which the glasses are held with regard to those in the lantern. Let us suppose that the thin glass plates forming the polariser in the top lantern of a bi-unial are placed north and south. If, now, the picture on the screen be looked at through the glasses held north and south, it is seen practically as clearly as if nothing were between the eye and screen. If, however, they are turned to east and west the picture disappears. Let the thin glass platcs forming the polariser in the bottom laniern be placed cast and west, .the polariscr in the top one remaining north and south. We have now two pictures on the screen. Let these be now looked at through the thin glass plates (the analyser), and it will be seen that when they are held north and south, the picture projected by the top lantern, in which the plates ave north and south, will be clearly seen, whilst the picture from the bottom lantern in which the plates arc east and west will have disappeared. Turn the plates to a position of cast and west aud the picture from the bottom lantern appears, and that from the top lantern disappears. If then we have a number of thin glass plates for each cye, the one lot being held north and south, and the other east and west, it is obvious that the former will allow only the picture of the top lantern to reach one eyc, and equally obvious that the latter will permit the picture from the bottom to reach the other eye, and that one only. The conditions, therefore, under which two fiat pictures can be made to appear with stereoscopic effect are fulfilled, and one picture only 101 is seen in relicf. The pictures, 10 feet in diameter, are projected upon a screen of calico covered with dead silver paper, the latter being a necessity as a linen screen or white wall destroys the polarisation. The system may appear to be a complex one from the cxplanation, in reality it is extremely simple. ‘The polarisers have simply to be placed in position, and the two lanterns turned on. There is no adjustment required, and the polarisers can be removed, when not needed, in a few seconds. The thin glasses or analysers through which the pictures are viewed, are mounted in a frame similar to an opera glass, but very much smaller and lighter. The tubes being but one inch and a quarter in length, and a trifle less than this in diameter. As the two pictures of a stereoscopic pair are dissimilar, they cannot properly be superimposed, and the effect as seen by the unaided eye is a confused picture double in parts. When looked at through the analysers one picture ard one only is seen, and that with the relief given by the stereoscope to a stereoscopic slide. Colored as well as ordinary photographs can be exhibited satisfactorily, and any number of observers can scc the pictures at the same time. In this description no attempt has been made to explain how a clear picture can be obtained after the rays have passed through a nurber of thin plates, nor how a clear view of the picture can be seen through analysers also made of thin glass plates. It must suffice how, to state that these difficulties have been overcome. ——'0 How | Warm my Saturator, By Epwp. Luckett. I po not consider that Mr. Pumphrey is correct when he says that saturators have had their day, as the best light can be obtained from them with a minimum of trouble and considerably less cost than with two gas cylinders. I give below the plan I adopt for warming My lantern box I use as a stand for the lantern, and place the saturator on an my saturator.