J. P. Cooke on the projection of Spectra. 245 
rest of the apparatus. I now use, however, a regulator better 
adapted to the purpose, made by J. Duboscq, of Paris. The 
negative pole, which is the lowest, is formed of asmall cylinder 
ing asmall cavity at the top to receive the 
fastened to the rim of a circular brass disk, which also hs hago 
four other similar cylinders. So that, by turning the disk, one 
after the other may be brought under the positive pole, and dif- 
ferent metals volatilized in the voltaic arc without further ad- 
justment. 
