You can make radium photographs, or skiagraphs as they are called, with any one of a number of radioactive substances and at a very small outlay.

59 A stereopticon is really two magic lanterns, but the word is now often used to mean a high-grade magic lantern.

60 Lantern slide plates can be bought at any photographic supply house.

The four most important radioactive substances, if we except radium itself, are black uranium oxide, pitchblende, thorium nitrate and uranium nitrate. You can buy any one of these substances in a glass stoppered bottle for $1.00 or the set of four for $3.50.61

61 The L. E. Knott Apparatus Co., Boston, carries these radioactive substances in stock.

While the radioactivity of these substances is low it is sufficient to make a shadow-picture - and this is all that an X-ray picture is - of a coin or other small object if it is laid on top of a dry plate sealed in a black paper envelope, which is opaque to the light.

That is, the coin is laid on the envelope containing the dry plate, and the bottle with the radioactive substance in it is laid on top of the coin. Let them remain undisturbed in this way for a couple of days and you will find on developing the plate a very good radiograph, or shadow picture of the coin as shown in Fig. 61.

Fig. 61. a photograph of a coin made with radium