[Illustration:  Duplicator Attached to a Camera]

The cardboard is cut triangular and attached to the front end of the camera as shown in Fig. 1 with a pin about 1 in. above the lens opening.  A rubber band placed around the lower end of the cardboard and camera holds the former at any position it is placed.  A slight pressure of the finger on the point A, Fig. 2, will push the cardboard over and expose one-half of the plate and the same pressure at B, Fig. 3, will reverse the operation and expose the other one-half.  Pins can be stuck in the end of the camera on each side of the lens opening at the right place to stop the cardboard for the exposure.  With this device one can duplicate the picture of a person on the same negative.  —­Contributed by Maurice Baudier, New Orleans, La.

** Optical Illusions [364]

The accompanying sketch shows two optical illusions, the first having a perfect circle on the outside edge

[Illustration:  The Two Illusions]

appears to be flattened at the points A, and the arcs of the circle, B, appear to be more rounding.  In the second figure the circle appears to have an oval form with the distance from C to C greater than from D to D. A compass applied to the circles in either figures will show that they are perfectly round.  —­Contributed by Norman S. Brown, Chippewa Falls, Wis.

** Use of Kerosene in Polishing Metals [364]

Anyone who has polished a flat iron or steel surface with emery cloth knows how soon the cloth gums and fills up.  The cloth in this condition will do little or no cutting.  A simple remedy for this trouble is to use kerosene on the surface.  The oil floats away a large part of the gumming substance and leaves the emery cloth sharp and clean to do the best work, also, it seems to act as a lubricant to keep particles of metal from collecting on the cloth and scratching or digging in the surface of the metal.  A very light lard oil is equally good for this purpose, but not always easily obtained.  A surface polished where oil or kerosene is used does not rust so easily as one polished dry, for the reason that a little oil remains on the metal.

Kerosene is the best to use on oil stones, being better than heavier oil.  This oil readily floats away all particles of the feather edge that are liable to become loosened and forced into the stone.  These particles of metal when stuck to the stone are the cause of spoiling it, as well as nicking the tools that are being sharpened.  Keep the surface of the stone well oiled at all times to make the cutting free.  —­Contributed by Donald A. Hampson, Middletown, N. Y.

** How to Make Lamps Burn Brightly [364]

For a good, steady light there is nothing better than a lamp, but like most everything it must have attention.  After cleaning well and fitting it, place a small lump of camphor in the oil vessel.  This will greatly improve the light and make the flame clearer and brighter.  If there is no camphor at hand add a few drops of vinegar occasionally.