104 ANNUAL OF SCIENTIFIC DISCOVERY. 



bling a galvanometer ; the galvanic current, passing through a coil of 

 wire, magnetizes a bar of soft iron, which is passed through the coil ; 

 and in proportion as the current is strong or feeble, the magnetized 

 bar rises or falls. When the current is in excess, it actuates an 

 escapement, and the two electrodes are drawn to the required distance 

 apart; and when the current passing is less than the regulated quanti- 

 ty, the motion is reversed, and the electrodes are drawn nearer to- 

 gether. 



Thus the light is rendered steady and constant, while no more of 

 the fluid is allowed to pass than is developed in light, effecting a great 

 economy of battery-power. To prevent injurious vibrations or sud- 

 den movements of the iron bar, it is provided with a rack, wheel- 

 work, and fly. Another improvement consists in giving the upper 

 electrode the form of a circular disk made to revolve slowly in con- 

 tact with a fixed scraper, which keeps the edges clean and i'ree from 

 the particles of carbon projected upon it from the lower electrode. 

 The carbon is prepared by forming a powder of charcoal into paste 

 with melted brown sugar, pressing it into iron moulds, and baking it 

 in the moulds at a red heat, and afterwards in a crucible at a white 

 heat. 



There have been several public exhibitions of this light, all of which 

 have been successful. In one case it was exhibited in the large 

 rooms in Hanover Square, London. The rooms were, as usual, lighted 

 with chandeliers of wax candles, with a considerable number of oil- 

 lamps ; the total amount of light being considered to be equal to 200 

 or 300 wax candles. On the lecture-table was the light apparatus, 

 covered with a tall glass shade. All things being made ready, the 

 galvanic circuit was completed, and in a few seconds the whole apart- 

 ment was filled with such a blaze of diffusive light, as caused the now 

 dimly burning candies and lamps to assume the muddy and lack- 

 lustre aspect they bear in ordinary sunlight. Every object in this 

 large room was brilliantly illuminated, and as an assistant turned the 

 light on and off at pleasure, the transition was as violent as from 

 broad clay to evening twilight. The paintings on the ceiling were 

 finely displayed ; and, what was very remarkable, the tone of the 

 colors was precisely similar to that which they are seen to possess 

 in daylight. All the delicate intershadings of the yellows, grays, 

 flesh-tints, and even of greens and blues, were brilliantly defined, and 

 in all respects conveyed the daylight impression to the eye. The light 

 was about equal to that of 700 or 800 standard wax candles, yet a 

 lady's bonnet might have covered the entire apparatus; and the actual 

 source of light did not occupy an area of more than an inch in every 

 direction, if so much. The rays were then concentrated by a powerful 

 lens, and directed upon some pictures, which were placed for the pur- 

 pose on the side of the room, and the colors could be as clearly seen 

 as by the light of the sun. 



By means of a glass prism, a spectacle yet more beautiful was 

 shown : this was the display of the prismatic spectrum, the entire 

 number of the rays being present, and in brilliancy not to be distin- 

 guished from the same as shown by the decomposition of the true 



