484 Dr. R. Bornstein on the Influence of Light 



wire. The bridge-wire was of German silver, 1150 millimetres 

 long ; and to each end there was attached a spiral of copper 

 wire. By this arrangement the sensitiveness of the apparatus 

 became very much greater than it had been when the German- 

 silver wire, originally intended for this purpose, was alone 

 employed. To prevent any changes of temperature by radia- 

 tion, the two spirals were covered with paper shades during 

 the experiments, and the German-silver wire was protected 

 against any heat radiated from the observer's body by a strip 

 of wood ; and before each observation its temperature was 

 rendered uniform by fanning it with a piece of cardboard. 

 The true position of the slider was calculated from two posi- 

 tions differing by one millimetre, on the assumption that 

 within these limits the variations in the deflection of the gal- 

 vanometer and the position of the slider were proportional to 

 one another. In each of these two positions of the slider, the 

 first deflection of the galvanometer-needle was read off for 

 both directions of the battery-current; and then the second 

 commutator was made use of, and the whole repeated. Eight 

 readings were consequently taken for each determination of 

 the resistance, which was calculated from them by means 

 of a formula determined empirically for this piece of appa- 

 ratus. 



Experiments were first made upon two very thin platinum 

 wires, which were obtained from what is called Wollaston 

 wire, by dissolving off the outer layer of silver with nitric 

 acid. To hold them a box-shaped enlargement was made at 

 that part of the telescope, belonging to the spectroscope em- 

 ployed, where the cross-wires are usually situated. This por- 

 tion, which was of brass, had four slightly conical holes at 

 right angles to the axis of the telescope ; and brass plugs were 

 fitted, air-tight, into these holes, and insulated by means of 

 vulcanite collars. The plugs were connected on the inside 

 with platinum clips, all insulated with vulcanite ; and between 

 these clips the two platinum wires were stretched. Their 

 ends were soldered to the clips ; and they were placed in the 

 same plane and parallel, symmetrically situated, and at right 

 angles to the axis of the telescope ; and by means of the con- 

 nexions already described they could be separately joined up, 

 from the outside, in an electric circuit. This box containing 

 the wires was closed on the side towards the object-glass by 

 a piece of plate glass, and on the opposite side by an eye-lens, 

 so that it could be connected with an air-pump and exhausted. 

 It appeared afterwards that no change in the experimental 

 results was produced when this space was exhausted to a 

 pressure of 15 millimetres of mercury ; and consequently this 



