To direct the flash of the Mirror.—­There are makeshift ways of directing the flash of the mirror; as, by observing its play on an object some paces off, nearly in line with the station it is wished to communicate with.  In doing this, two cautions are requisite:  first, the distance of the object must be so large compared to the diameter of the mirror that the play of the flash shall appear truly circular and exactly like a faint sun (see preceding paragraph):  secondly, be careful to bring the eye to the very edge of the mirror; there should be as little “dispart” as possible, as artillerymen would say.  Unless these cautions be attended to very strictly, the flash will never be seen at the distant station.

[Sketch of arrangement as described below].

An object, in reality of a white colour but apparently dark, owing to its being shaded, shows the play of a mirror’s flash better than any other.  The play of a flash, sent through an open window, on the walls of a room, can be seen at upwards of 100 yards.  It is a good object by which to adjust my hand heliostat, which I describe below.  Two bits of paper and a couple of sticks, arranged as in the drawing, serve pretty well to direct a flash.  Sight the distant object through the holes in the two bits of paper, A and B, at the ends of the horizontal stick; and when you are satisfied that the stick is properly adjusted and quite steady, take your mirror and throw the shadow of A upon B, and further endeavour to throw the white speck in the shadow of A, corresponding to its pin-hole in it, through the centre of the hole in B. Every now and then lay the mirror aside, and bend down to see that A B continues to be properly adjusted.

Hand Heliostat.—­Some years ago, I took great pains to contrive a convenient pocket instrument, by which a traveller should be able to signal with the sun, and direct his flash with certainty, in whatever direction he desired.  I did so in the belief that a signalling power of extraordinary intensity could thus be made use of; and, I am glad to say, I succeeded in my attempt.  I at last obtained a pretty pocket instrument, the design of which I placed in the hands of Messrs. Troughton and Simms; and upon the earlier models of which I read a paper before the British Association in 1858.  I called it a “hand heliostat.”  I always carry one when I travel, for it is a continual source of amusement.  The instrument is shown in fig. 1 (p. 280), and its principle is illustrated by fig. 2.  The scale is about 2/3.

E is the eye of the signaller; M the mirror; and L, S, fig. 2, a tube containing at one end, L, a lens, and at the other, S, a screen of white porcelain or unpolished ivory, placed at the exact solar focus of L:  a shade, K, with two holes in it, is placed before L. Let R, r, be portions of a large pencil of parallel rays, proceeding from any one point on the sun’s surface, and reflected from the mirror, as R’ r’ (fig. 2).  R’ impinges