From a manufacturer of optical goods purchase a pair of four-and-a-half-inch diameter condensing-lenses mounted in a tube, and a quarter-size four-inch back-focus double-objective lens mounted in a tube with rack and pinion adjustment.
Fig. 15. Fig. 16.
The condenser-lenses should occupy the hole in the front of the box, where the tube can be firmly held in place by the inside edge of the hole. The objective lenses are to be attached to the face of the front board in the position shown in Fig. 16; and by means of the sliding-board and bellows the lenses can be adjusted to meet any short-focus range—that is, a distance of fifteen or twenty feet from the sheet on which the picture is thrown.
For use in small rooms a duplex-burner, or central-draught lamp, can be used, and a plan showing the position of lamp, reflector, and lenses is shown in Fig. 15.
A represents the reflector placed behind a lamp having duplex wicks. The centre of the reflector should be directly in line with the centre of the lenses, and the lamp must be placed so that a portion of the flame, three-quarters of an inch above the top of the wicks, will line with centre of reflector and lenses. The light is brightest in oil-lamps, from three-quarters to an inch above the wicks, and, as only a small portion of the light is of use, the most brilliant part of the flame should be employed for projection through the slide and lenses.
B B are the wicks of the lamp, and the fount should be placed so that an imaginary line drawn through the centre of reflector and lenses would pass through the forward edge of one wick and the rear edge of the other. By arranging the wicks at the angle shown on the plan, the greatest advantage can be had from the light.
The positions of the condensing-lenses are shown at C C, and D represents a slide in position at the front of them. E E gives the position of the objective lenses, and the dotted lines F, G, and H, drawn from the reflector edges crossing at the light, passing through the condensers and slides, and out through the objective lenses, show the manner in which the rays of light are collected and thrown through the transparent picture and lenses, and so on through space, until it rests against the large sheet.