Reflecting the Sun with a Mirror.—­To attract the notice of a division of your party, five or even ten miles off, glitter a bit of looking-glass in the sun, throwing its flash towards where you expect them to be.  It is quite astonishing at how great a distance the gleam of the glass will catch the sharp eyes of a bushman who has learnt to know what it is.  It is now a common signal in the North American prairies. (Sullivan.) It should be recollected that a passing flash has far less briliancy than one that dwells for an appreciable time on the retina of the observer; therefore the signaller should do all he can to steady his aim.  I find the steadiest way of holding the mirror is to rest the hand firmly against the forehead, and to keep the eyes continually fixed upon the same distant object.  The glare of the sun that is reflected from each point of the surface of a mirror forms a cone of light whose vertical angle is constant, and equal to that subtended by the sun.  Hence when a flash is sent to a distant place, the size of the mirror is of no appreciable importance in affecting the size of the area over which the flash is visible.  That area is the section of the fasciculus of cones that proceed from each point of the mirror, which, in the case we have supposed, differs immaterially from the cone reflected from a single point.  Hence, if a man watches the play of the flash from his mirror upon a very near object, it will appear to him of the shape and size of the mirror; but as he retreats from the object, the edges of the flash become rounded, and very soon the flash appears a perfect circle, of precisely the same apparent diameter as the disc of the sun:  it will, in short, look just like a very faint sun.  The signaller has to cause this disc of light to cover the person whose notice he wishes to attract.  I will proceed to show how he can do so; but in the mean time it will be evident that a pretty careful aim is requisite, or he will fail in his object.  The steadiness of his aim must be just twice as accurate, neither more nor less, as would suffice to point a rifle at the sun when it was sufficiently obscured by a cloud to bear being looked at:  for the object of the aim is of the same apparent size, but a movement of a mirror causes the ray reflected from it to move through a double angle.

The power of these sun-signals is extraordinarily great.  The result of several experiments that I made in England showed that the smallest mirror visible under atmospheric conditions such that the signaller’s station was discernible, but dim, subtended an angle of only one-tenth of a second of a degree.  It is very important that the mirror should be of truly plane and parallel glass, such as instrument-makers procure; the index glass of a full-sized sextant is very suitable for this purpose:  there is a loss of power when there is any imperfection in the glass.  A plane mirror only three inches across, reflects as much of the sun as a globe of 120 feet diameter; it looks like a dazzling star at ten miles’ distance.