6 NO REACTION BETWEEN A MAGNET OR VOLTAIC CURRENTS AND LIGHT. 



was separated from the others by passing through a slit in a metallic screen, and half 

 the needle shielded from its action by a piece of paper. After two hours' exposure to 

 the sun, it was suspended again in the exhausted receiver, but still showed no token of 

 polarity ; it was then exposed to the other rays successively, with the same result. The 

 needle was now slightly touched, and, slowly vibrating, arranged itself along the mag- 

 netic meridian. The first vibration was performed in a semicircular arc, and the num- 

 ber of vibrations performed during one hundred oscillations of a seconds' pendulum was 

 twenty-seven. But, after four hours' exposure to the violet ray, as before, no evidence 

 of any change, either increasing or diminishing the number of oscillations, could be 

 gained. A column of violet light, passing through a disk of stained glass, was con- 

 centrated on one end of a sewing needle by means of a lens, without producing any 

 change in the number of vibrations it made in one minute. This needle, on some oc- 

 casions, however, would give unequal results ; when its first vibration was performed 

 in a semicircle, the number varied from forty-one to forty-three in sixty seconds. On 

 vibrating it in vacuo, its results uniformly gave the latter number very nearly. 



11. The position of the needle to the incident ray is not of any consequence, whether 

 it receives it obliquely in the direction of the light or across it. If soft iron be substi- 

 tuted for steel, the results are still negative, even if the needle be arranged in the mag- 

 netic meridian, the line of dip, or any other position. I therefore come to the conclu- 

 sion that the violet ray, as developed by a prism of English flint glass, possesses no in- 

 fluence on the magnetic needle, and all the other rays are equally inert. 



12. But, as Mrs. Somerville found that a needle placed under a piece of glass or blue 

 riband, having half its length protected by paper, became, in a short time, magnetic, I 

 tried the same experiment, but, in every instance, failed in making the needle magnetic. 

 When suspended by a silk fibre in vacuo, needles showed no disposition to arrange 

 themselves in any particular line, and when they came to rest, they were found cutting 

 the magnetic meridian at every angle, although the temperature of the sunbeam to which 

 they had been exposed on one occasion was 124 F. Great care was taken to ascer- 

 tain the previous non-magnetic state of the needles, and they were suspended by a fibre 

 without torsion. To ascertain whether anything was due to the nature of the medium, 

 I substituted prisms of water, alcohol, spirit of turpentine, and other essential oils, with 

 the same results. 



13. These are the means by which, it is said, the magnetism of light was first dis- 

 covered ; but there are much more delicate methods of detecting such an action if it 

 existed, and to them, in the next place, I resorted. For, if the violet or other rays of 

 light exercise an influence on the magnetic needle, that action must be mutual between 

 them, and the light, in its turn, should suffer a derangement. To ascertain this, I ad- 

 mitted a divergent beam of light through a hole in the shutter of a dark room ; the cone 

 of luminous matter at its apex was about T ' T th of an inch in diameter, and a hair or oth- 

 er filament held in it exhibited the phenomena of diffraction, the colours being received 

 into the eye by a lens. Across this beam a silver wire was adjusted, each of its ex- 

 tremities connected with cups of mercury, which communicated with the poles of a 

 voltaic battery. It was expected that, if there was any action between a magnetic fila- 



