We have had three diagrams engraved of the apparatus necessary for producing dissolving views on a large scale suitable for a lecture-room, or exhibition of any kind.
Our first figure (fig. 4) represents the form of lantern used at the Royal Polytechnic Institution, London. It consists of a box (A) with a projecting part(B), having an opening (o) between the back part and the condensers of the two lanterns contained in the box. The painted slides are inserted at (o), and thus pass between the light and the condensers or lenses. In this apparatus the lenses are made of the best glass, so as to avoid achromatic refraction. The top of the box is fitted with two chimneys (G G) made of japanned iron to allow the smoke and heat to escape. In front of the box we observe the barrels of the lanterns (EE), with the rack-work which regulates the focus by means of a screw (c) placed above them. The box containing the two lanterns is placed upon ft firm stand (D D), having a slide passing underneath, which is fitted at one end with an upright piece having the dissolving fans placed on either side of a central point (F). By this arrangement the fans can be raised or depressed at the will of the exhibitor, and retained in their position by means of the screw (H), and they may also be made to advance or recede from the nozzles of the lanterns by means of the slide which passes under the table.
In shutting off the light, it is necessary to pay attention to the following observations : When the light is thrown from one lantern we obtain a large circle or disc of light thrown upon the screen; and our object in exhibiting is always to have a disc of this size, or nearly so, reflected upon the screen; therefore, in shutting off the light, it will be necessary to adjust the fans, so that the under part of one lens is only obscured as much as the upper part of the other is displayed. By this means we are enabled to preserve the brilliancy of the views and prevent the disc being irregular and dusky at the upper and lower parts. As it is sometimes necessary to use both lanterns at the same time, the fans or dissolvers are movable.
The light used in these lanterns is supplied by the combustion of oxygen and hydrogen gases in a combined state, the flame being thrown upon a cylinder of lime, so as to produce the Drummond Light; and in order that the manner in which this is done may be perfectly understood, we have had a diagram engraved.
It represents the interior of the box and the back part of the condensers (B B). About 8 inches from the condensers are cylinders of lime placed upon a pivot which has a small cog-wheel at the lower part of it, and which is connected with another wheel at the lower part of the key (K), used to wind up the machinery. The object of employing this machinery is to cause the lime cylinders to revolve slowly upon their axes, so as to expose a fresh surface to the action of the flame, which is so intense that it will even melt a diamond. Close to the lime cylinders you will see the blow-pipes by which the gases are thrown upon the lime; these issue from the receivers (D D), where the gases are mixed after being supplied by the pipes (E E) connected with large caoutchouc bags (fig. 7, F,) placed between press-boards, which are loaded with weights to force the gas out of the bags. After the gases have been mixed, they may be safely ignited at the end of the blowpipe, and the flame allowed to play upon the cylinder; but you should be careful not to allow a flame to approach these gases in a mixed state, without they are connected with a receiver or a Hemming's safety-tube, for if this precaution is neglected, a very dangerous explosion will ensue. It is the method now generally employed to prevent accidents of this kind, and one that is extremely simple and valuable. A square receiver of brass (K) is filled with fine brass wire, which is pressed tightly together, so that when the gases enter the receiver by the tubes (O and H), which are connected with the caoutchouc bags containing the oxygen and hydrogen gases, they then pass through the spaces between the brass wires, which are now, in fact, narrow tubes. After the gases have been mixed, they pass out of the receiver and through the blow-pipe (B), to be thrown upon the lime cylinder, and thus produce a most intense, pure, and beautiful light, well-known as the Drammond Light.
The lime cylinders should be wrapped in paper singly, and the whole kept in bottles with well-greased stoppers.
To make the lime cylinders, procure a piece of chalk or limestone, and cut it into pieces about 1 1/2 inches long, and 3/4 inch in diameter, and as round as you can; then drill a hole through the centre of each, in the long axis ; and, having placed them in a crucible in the centre of a good fire, keep them red-hot for about four hours. Cool them gradually, and wrap in paper as soon as possible. A convenient form of dissolving apparatus for a private exhibition, and also for lecturers who have to travel from town to town, is that shown in fig. 7. It consists of a stand (A) with folding tripod legs (E E E E), and having a slide underneath, and, as in the former one, supplied with dissolvers or fans (D). The lanterns (BL) are made of mahogany, with japanned iron tops, having a place (S) for the reception of the slides, before which are the moveable tubes (C) with the necessary lenses. A caoutchouc bag (F) fitted with a stopcock, and flexible or vulcanized india-rubber-tube (0) unions, and press-boards, is tilled with oxygen gas; the boards are loaded with weights (W) to maintain an equal pressure of the gas, and another similar bag (G) filled with hydrogen gas is also loaded with weights, and connected with the apparatus by a flexible tube (H). This apparatus is so constructed that it may be packed away with the tubes, pressure-boards, lanterns, slides, etc, into a comparatively small space; and as it may be exhibited with as much ease as an ordinary lantern, it is extremely useful for the general purposes of schools, lecturers, and families. The small magic-lanterns may be procured of almost any optician, and vary in Price. Magic lanterns of every description, with a large and well-selected assortment of Sliders, beautifully painted,Slides illustrating the most prominent events in Scripture, and particularly appropriate for Sunday-school exhibitions. Also, Astronomical, Temperance, and Humorous designs. They can be found at the store of McAllister and Bro.,728 Chestnut-street, Philadelphia.