The author of this paper has, for many years, used a plane mirror at 45 degrees with the table, to demonstrate the properties of rays and beams of light. A horizontal projection of rays must be viewed from above in the absence of a mirror. If a mirror is used, the horizontal layout is seen in a vertical plane in the mirror.

It is only recently, as the result of an A.R.P. demonstration on incendiary bombs, that the effectiveness of the mirror method of demonstration has become apparent as well as the frequent and diverse occasions in which it is useful. In the demonstration referred to, some houses, farm buildings, hay stacks, and the like were exhibited as models on a table. The point of striking of an incendiary bomb was registered by the lighting up of a small flash lamp. Scarcely anyone in the large audience could see what was happening. A mirror at 45 degrees would have rendered the whole proceedings visible to everyone.