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.

Attention was called to the graphical representation of a geometric progression by W. J. Dobbs, Gazette, IX, p. 245, as in Fig. 1. Such illustrations,

together with numerical examples, are sufficient for School Certificate examination purposes. The following considerations are intended to fill up time after the examination.

To the ordinary student simple harmonic motion is the motion of the bob of a swinging pendulum, of a mass at the end of an oscillating spring, or of a point on the rim of an oscillating fly-wheel; by definition, a body or particle is moving in simple harmonic motion if its acceleration is proportional to its distance from a fixed point, and directed towards that point.

A School time-table is one of the means by which we give opportunities to all pupil, a member of a class in a school, to study during a definite time (a school period) and place (a desk in a room: see below) a particular lesson, one of a prescribed syllabus in a subject, with the help of a particular teacher, member of the school staff. It thus involves a process of grouping the five variables—period, teacher, pupil, room and subject—according to certain desirable objectives and rules.