642 SEC. 15. GEOGRAPHY. 



If held in such a position that the sun's light falls in the solid angle 

 between the face of the mirror and the two small polished surfaces, a portion 

 of the sun's light that falls upon the face of the mirror is refracted at the 

 first surface, reflected internally at each of the small surfaces, and finally 

 emerges through the space from which the silver has been removed, in a 

 direction parallel to, but opposite to, that in which the reflected light travels 

 from the large plane of the mirror. 



Hence, any point with which the faint image of the sun appears to 

 coincide will receive the light of the sun reflected from the mirror. 



3136. Sun Signals, for the use of travellers. 



Francis Gallon, F.R.S. 



The difficulty in sun-signalling is to direct the flash aright. The rays of 

 the sun are reflected from a mirror, in a cone of light precisely similar to that 

 which reaches it, the mirror itself (whose size may be disregarded) being the 

 apex of the latter cone, and the sun's disc its base. It follows that to the 

 signaller, whose eye is near the mirror, the place where the cone of reflected 

 rays falls on the distant landscape would always appear to him as a disc of 

 precisely the same shape and size as the sun itself. In other words, his 

 accuracy of aim must be within 30 minutes of a degree. In the author's 

 heliostat an image of the sun is produced, which overlies the area on which 

 the flash of the mirror falls. A lens is fixed in the instrument at right angles 

 to the line of sight ; half of the lens lies within the tube through which the 

 observer looks, and occupies a portion of his field of view, the other half is 

 external to his field of view ; it projects beyond the side of the eye tube, and 

 receives the flash of the mirror. The mirror turns on an axis attached to the 

 tube, which allows it movement in one direction, while the rotation of the 

 entire instrument in the hand gives movement in the other. When the mirror 

 is so adjusted that the reflected (parallel) rays from any one point of the 

 sun's disc impinge on the lens, they are brought to a focus on the screen, and 

 form a minute speck of light upon it. Rays radiate from this speck in all 

 directions, and those that strike the part of the lens inside the eye tube are 

 reduced by its means back again to parallelism with the rays that originally 

 left the mirror. Consequently the eye, looking down the tube, sees a bright 

 speck through the lens, which it refers to the same distant point in the land- 

 scape seen to the side of it as that to which the unobstructed rays from the 

 mirror are being flashed. If a telescope be fitted to the tube the speck would 

 overlie the spot on the landscape. Now what is true for any one point in the 

 sun's disc is true for every point, therefore the signaller sees a luminous disc 

 in his field of view, and this exactly overlies the locus of the flash. By gently 

 rotating the hand the image can be made to cover or to forsake any point in 

 the landscape that may be desired, and when that is done an observer sta- 

 tioned at that point will see a succession of flashes. Morse's alphabet can be 

 adopted. A flash passing through a square hole of only one-third of an inch 

 in the side is visible to the naked eye at a distance of 10 miles, if the back- 

 ground be dull and the air perfectly clear. 



3137. Optical Telegraph, by Colonel Laussedot, composed 

 of a transmutor and of a receiver, with their stands. 



Colonel Laussedot, Paris. 



3138. Models illustrating two methods of verifying Sex- 

 tants employed at the Kew Observatory : 



1st. By flashing the sun's rays to distant mirrors, whereby stars 

 of light were visible to the operator at the testing table. 



