768 SEC. 15. GEOGRAPHY. 



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. The principle of this heliostat was 

 described in a memoir read before the British Association in 1858. 



3137. Optical Telegraph, by Colonel Laussedat, composed 

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



Colonel Laussedat, Paris. 



The principle of this optical telegraph is due to Professor Mauras. Several 

 other scientific men, during the siege of Paris in 1870-7], and since have 

 assisted, under the direction of Colonel Laussedat, in modifying and per- 

 fecting the construction of the original apparatus. The model exhibited is the 

 one which Colonel Laussedat proposed for country telegraphs and for the 

 application of luminous signals to geodesy. When the sky is cloudless, 

 recourse can be had to the rays of the sun ; when the case is otherwise, one 

 must be contented with the flame of the petroleum lamp, of which the range 

 is far more limited. By night its range increases considerably, so much so 

 that when the two small lanterns are exposed it is possible to send and 

 receive signals at a distance of more than 40 kilometres ; the communication 

 takes place by means of luminous flashes and eclipses effected by a small 

 screen, worked by the sender of the message. The Morse alphabet can be 

 used by using short flashes for the points, and longer flashes for the lines. 

 The working of the instrument can quickly be learnt, and this very simple 

 apparatus is most useful in cases where the electric telegraph is wanting. 



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. 

 This was ceased to be employed on account of the rarity 

 of sunshine. Designed by F. Galton, F.R.S. 



2nd. A system of five collimators, fixed firmly to a wall on a 

 circular arc, arranged so as to send parallel rays across 

 the testing table at known angles. Designed and con- 

 structed by J. Cooke, and described in the Proceedings 

 of the Royal Society, vol. XVI., page 2. 



Kew Committee of the Royal Society. 



3099f. Prismatic Compass, with improvement of ring to 

 hold more safely in the hand. Henry Porter. 



