VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 231 



the handle rD is fastened. A second string is fastened to the slider, at the right 

 side, and goes towards the centre, where it passes over a pulley n, and the 

 weight w, which is suspended by the end of this string, returns the slider to- 

 wards the centre, when a contrary turn of the handle permits it to act. 



a and b are 2 small lamps, 2 inches high, 1^ in breadth, by 1-J. in depth. The 

 sides, back, and top, are made so as to permit no light to be seen, and the 

 front consists of a thin brass sliding door. The flame in the lamp a is placed -^ 

 of an inch from the left side, T V * rom tne front, and half an inch from the 

 bottom. In the lamp b it is placed at the same height and distance measuring 

 from the right side. The wick of the flame consists only of a single very thin 

 lamp-cotton thread ; for the smallest flame being sufficient, it is easier to keep it 

 burning in so confined a place. In the top of each lamp must be a little slit, 

 lengthways, and also a small opening in one side near the upper part, to permit 

 air enough to circulate to feed the flame. To prevent every reflection of light, 

 the side opening of the lamp a should be to the right, and that of the lamp b to 

 the left. In the sliding door of each lamp is made a small hole with the point of 

 a very fine needle just opposite the place where the wicks are burning, so that 

 when the sliders are shut down, and every thing dark, nothing shall be seen 

 but two fine lucid points of the size of 2 stars of the 3d or 4th magnitude. 

 The lamp a is placed so, that its lucid point may be in the centre of the circular 

 board where it remains fixed. The lamp b is hung to the little slider which 

 moves in the rabbet of the arm, so that its lucid point, in a horizontal position 

 of the arm, may be on a level with the lucid point in the centre. The move- 

 able lamp is suspended on a piece of brass fastened to the slider by a pin exactly 

 behind the flame on which it moves as a pivot. The lamp is balanced at the 

 bottom by a leaden weight, so as always to remain upright, when the arm is 

 either lifted above, or depressed below, the horizontal position. The double- 

 jointed handles rD, ep, consist of light deal rods, 10 feet long, and the lowest 

 of them may have divisions, marked on it near the end p, expressing exactly 

 the distance from the central lucid point, in feet, inches, and tenths. 



From this construction we see, that a person at a distance of 10 feet may 

 govern the 2 lucid points, so as to bring them into any required position south 

 or north preceding or following, from O to go , by using the handle p, and also 

 to any distance from -^ of an inch to 5 or 6 and 20 inches, by means of the 

 handle d. If any reflection or appearance of light should be left from the top 

 or sides of the lamps, a temporary screen, consisting of a long piece of paste- 

 board, or a wire frame covered with black cloth, of the length of the whole arm 

 and of any required breadth, with a slit of half an inch broad in the middle, 

 may be affixed to the arm by 4 bent wires, projecting an inch or 2 before the 



