158 ANNUAL OF SCIENTIFIC DISCOVERT. 



the screen, but the principle was the same. The present instrument dis- 

 penses with all fixture it is more portable than a ship's telescope, and as 

 manageable as a ship's quadrant, and may be made by a carpenter for 4s., 

 if he possesses a convex spectacle lens of short focus, and a piece of good 

 looking-glass. The looking-glass attached to the hcliostat is about three 

 inches by four and a half inches, and therefore capable of being seen at 

 distances, which may be calculated from the fact, that a mirror one inch 

 square is perfectly visible, in average sunny weather, at a distance of eight 

 miles, and that it shows as a brilliant glistening star at two miles. Before 

 describing its principle and action, it will be necessary to explain clearly the 

 peculiar characteristic of the reflection of the sun's rays from a mirror. If, 

 for instance, we take a small square looking-glass, and throw its flash upon a 

 wall two or three feet off, the shape of the flash will be little different from 

 that of the mirror itself, seen in perspective; but, if we direct it on an object 

 three or four yards off, the angles of the flash will appear decidedly rounded; 

 at twenty or thirty paces, it will appear fairly circular; and, if we can manage 

 to see it at fifty or one hundred yards (which can only be effected by selecting 

 some object to throw it on that is naturally of a light color, but lying under a 

 dark shade), it will appear like a mock sun, of identically the same shape and 

 size as the sun itself; and for all greater distances, the appearance remains the 

 same. That is to say, whatever may be the shape or size of the mirror, and 

 whatever the irregularity of the distant objects on which the flash happens 

 to be thrown, the shape and size of that flash, if it could be seen by the 

 signaller, would always appear to him as exactly that of sun. In fact, the 

 flash forms a cone of light, at the blunted apex of which arc the mirror ami 

 the signaller's eye, and whose vertical angle equals that of the sun's angular 

 diameter. Whoever is covered by the flash, sees the mirror, like a small 

 fragment of the sun itself, held in the hand of the observer, and the larger 

 the mirror, compared to the distance, the larger and the more dazzling docs 

 it appear. Now, the hand heliostat provides a bright appearance of the 

 sun, which, when the instrument is adjusted and looked through, overlays 

 the exact area which is covered by the flash of the mirror, which is attached 

 to its side. It is a perfect substitute for that mock sun which we can see at 

 fifty or one hundred paces distant, but which becomes too faint to be traced 

 much further. All we have to do, when we wish to send a flash to a dis- 

 tant object, is to make that image of the sun overlay the object, just as may 

 be done in rough sextant observations. The principle of the instrument is 

 extremely simple. A convex lens, of any focal distance (five inches is con- 

 venient), has a small screen attached to it, whose surface is at its focal 

 distance. The mirror is so placed that a small portion of its flash impinges 

 upon one end of the lens. The signaller's eye looks partly through the 

 other end of the lens, and partly free of it. Now the rays from any one 

 point of the sun's surface are converged by the upper part of the lens to a 

 bright point on the screen ; and those rays which radiate from that point and 

 impinge on the lower end of the lens, are brought back by means of it to a 

 state of parallelism with the rays that originally left the mirror. Conse- 

 quently, the signaller's eye sees the bright spot in the precise direction of the 

 vanishing point of the mirror's flash, and he can, by looking partly to the 

 side of the lens, refer it to some particular spot in the distant landscape. 

 But what is true for any one point on the sun's disk, is true for every point; 

 and, accordingly, we obtain a bright disk upon the screen, which appears of 

 exactly the same shape and size as the sun itself, and necessarily overlays 



