Page:Popular Science Monthly Volume 89.djvu/781

 �THE construction of a small stereop- ticon for use in the home or lecture room is an easy task for the work- man who possesses the usual tools, such as a hacksaw, small drill-press, taps, dies, etc. The design presented here- with is one well adapted to amateur construction, and in the finished instru- ment the builder will have one which incorporates, in a simplified manner, everything which makes for comfort and efficiency in operation. At the same time, it will cost little to build if the objective and condenser are purchased second hand.

From the illustrations, the reader will note that the finished instrument is of the conventional form with one or two exceptions. The usual bellows is sup- planted by telescoping c\lindcrs of metal ; the bellows is difficult to con- struct and, when made, it is no more effective than the de\ice shown in the drawing. The con\entional lamp-hood of sheet metal has been supplanted b\- an earthern hood made from a flower-pot; this construction permits the use of a simpK- made arc-lamp without gears or worms and, furthermore, it pre- \ents the heat of the arc fron) attacking the lamp parts. The heat is dissipated through an opening in the top, above the arc, and the condensing lens is protected by a ilisk of thin glass loosely clamped to the tront of the flower-pot hood. Every detail of the instrument and dimensions is shown in the drawings; it is therefore

���Fig. 1. conde

��unnecessary to repeat dimensions in the text of this article. Before starting the work, the builder should purchase the condensing lens, the objectixe and a slide carrier of the conventional sort. The condenser will cost, mounted as shown in Fig. I, about $2. The objectixe. Fig. 2 A, will cost from $3 to $10, depending upon its quality-, focal length, and condition, if second hand. The slide carrier of the sort shown in Fig. 3 costs about $5.

The focus of the objectixe will have an important bearing upon the length of the cylinders, C and D, Fig. 2, and the dimensions given are for a lens of lo-in. focus. This is an excellent size, as it throws a picture approximately 6 ft. square at a distance of 20 ft. from the screen. If the instrument is to be used in a smaller room, the objecti\"e should be of shorter focus. These oVjjective lenses are usually sup- plied in a mounting, with a flange and a knurled adjusting knob, which actuates a rack and pinion arrangement. The follow- ing table shows the sizes of pic- tures projected b\- lenses of vary- ing focuses at \arious distances. l"or example : With a lens having an equivalent focus of 7 in., a picture 6.5 ft. square would be projected if the machine were 15 ft. from the screen; 8.5 ft. square at 20 ft., etc.

The table is prepared as an aid to careful selection of a lens suitable to the room in which the stereopticon views are to be exhibited.

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