[Illustration:  Arrangement of Prisms]

dull black.  The paper which comes around plates answers nicely.  If the ring which slips over the lens mount is lined with black velvet, it will exclude all light and hold firmly to the mount, Place over lens, stop down well after focusing, and proceed as for any picture.

** Another Electric Lock [110]

The details of the construction of an electrically operated lock are shown in the illustration.  When the door is closed and the bolt A pushed into position,

[Illustration:  Simple Electric Lock]

it automatically locks.  To unlock, push the button D, which act will cause the electromagnet to raise the latch C, when the bolt may be drawn and the door opened.  —­Contributed by A. D. Zimmerman, Boody, Ill.

** How to Mix Plaster of Paris [110]

For the mixing of plaster of Paris for any purpose, add the plaster gradually to the water, instead of the contrary, says the Master Painter.  Do not stir it, just sprinkle it in until you have a creamy mass without lumps.  Equal parts of plaster and water is approximately the correct proportion.  The addition of a little vinegar or glue water will retard the setting of the plaster, but will not preserve its hardening.  Marshmallow powder also retards the setting.  In this way the plaster may be handled a long time without getting hard.  If you wish the plaster to set extra hard, then add a little sulphate of potash, or powdered alum.

** Enlarging with a Hand Camera [111]

Everyone who owns a hand camera has some pictures he would like enlarged.  It is not necessary to have a large camera to do this, as the process is exceedingly simple to make large pictures from small negatives with the same hand camera.

A room from which all light may be excluded and a window through which the light can enter without obstruction from trees or nearby buildings, with a shelf to hold the camera and a table with an upright drawing-board attached, complete the arrangement.  The back is taken out of the camera and fitted close against the back of the shelf, which must be provided with a hole the same size and shape as the opening in the back of the camera.  The negative used to make the enlarged print is placed in the shelf at A, Fig. 1.  The rays of the clear, unobstructed light strike the mirror, B, and reflect through the negative, A, through the lens of the camera and on the board, as shown in Fig. 2.  The window must be darkened all around the shelf.

After placing the negative and focusing the lens for a clear image on the board, the shutter is set and a bromide paper is placed on the board.  The paper is exposed, developed and fixed by the directions that are enclosed in the package of bromide papers.

[Illustration:  Making Large Pictures with a Small Camera]

** Positioning A Hanging Lamp [111]

Don’t pull a lamp hung by flexible cord to one side with a wire and then fasten to a gas pipe.  I have seen a wire become red hot in this manner.  If the lamp hung by a cord must be pulled over, use a string.