Page:Proceedings of the Royal Society of London Vol 60.djvu/400

Rh current could be supplied to the two electric lamps in the followingmanner :_During half a turn of the commutator, no current to either lamp; during the succeeding one-sixth of a turn, current to the interior lamp -only; during the remaining one-third of a complete turn, current to the exterior lamp only.

Starting with darkness, and turning the commutator quickly through 180°, the observer saw, as soon as the interior lamp was lighted, the shadow of the tinfoil, which was, as usual at the initial stage, of a bright red hue ; but a small fraction of a second later, before it had time to lose its redness and become black, the image was obliterated by a flood of light from the exterior lamp, while at the same moment the other lamp was extinguished.

When the commutator was caused to make four or five turns per second, the image of the tinfoil was almost continuous, and was at once recognised by inexperienced observers to be red.* This experiment was repeated in another form, the arrangement being such that the light of two lamps was interrupted by screening, instead of by breaking the current; the changes in the illumination could thus be made more rapidly.

Two black cardboard disks, from each of which a sector of 60° had been cut out, were mounted 3 | in. (9 cm.) apart at the ends of a horizontal axle, being so fixed that the posterior edge of the openingin one of the disks was exactly opposite to the anterior edge of that in the other. Between the disks, and in a parallel plane, was suspended a sheet of wflnte paper, across the middle of which a narrow strip of tinfoil was gummed. Two clear glass electric lamps were placed near the outer faces of the disks at the same height as the axis, the incandescent filaments being directed horizontally. To an observer looking at the plain side of the paper across the edge of one of the disks, while they were rotating slowly in the proper direction, the paper first appeared dark all over, then it was illuminated from behind by one of the lamps, the dark strip becoming visible; finally, it was illuminated from the front by the other lamp, and the strip could no longer be seen. When the angular velocity was sufficiently increased, the strip was seen continuously, or nearly so, and its colour was, as before, bright red. attending sudden Changes o f Illumination. 373

From a disk of white cardboard 6 in. (15 cm.) in diameter a sector of 60° was cut out; the remainder of the disk was divided into two 8-candle power and have very thin filaments, the efficiency being 2*5 watts per e.p. They were worked at a pressure of 6 per cent, above their marked voltage, and the incandescence responded very quickly to the current.
 * The lamps used in this experiment were made to my order. They are of