Transparent Bodies by tlie action of Magnetism. 485 



constant and variable ; besides, all the external actions of a maa;- 

 net, its magnetizing action, its inductive action, its action on 

 the element of a current, depend solely upon the action which it 

 would exercise upon a molecule of magnetic fluid. It is then 

 probable, that the action, in virtue of which it developes the rota- 

 tory power in transparent substances, depends solely upon the 

 same quantity. If it be so, we need no longer trouble ourselves 

 about the distribution of the free magnetism of the electro- 

 magnet ; no longer inquire separately into the influence of the 

 variations of the intensity of magnetization, and the influence of 

 the variations of distance. We shall measure, on the one hand, 

 the optical action of the transparent substance, and on the other, 

 a magnitude which at the same time represents in every point 

 the effect of the variations of distance and the effect of the varia- 

 tions of intensity; we shall then have taken account at once 

 of the influence of the two causes, and we can determine 

 the elementary law of the phenomena without any of those 

 arbitrary and inexact hypotheses to which we should have been 

 necessarily conducted, if, for instance, we had wished to deduce 

 the law relative to the influence of the distance in experiments 

 where it would have been necessary to vary the distance of the 

 transparent body from the extremity of one of the branches of 

 the electro-magnet. 



Experiment has, as we shall see, confirmed this supposition] 

 and to obtain the elementary law which was sought, it has been 

 sufficient to measure simultaneously the optical phasnomena and 

 the magnetic action. 



The measurement of the optical phsenomena has only been an 

 application of methods known to all physicists, employed in the 

 form which appeared to me most convenient for the peculiar 

 conditions of my experiments. 



A pencil of solar light, reflected by a heliostat into the dark 

 chamber where I experimented, was polarized by passing through 

 a NichoFs prism fixed at the entrance of the canal which passed 

 through the electro-magnet in the direction of its axis. At the 

 other extremity of the same canal the pencil of light encountered 

 a very narrow diaphragm, and the portion which emerged fell 

 upon the analysing apparatus placed at some distance. This 

 apparatus, constructed by M. Brunner, was composed of a small 

 telescope having an analysing prism placed before the object- 

 glass, and capable of turning about its axis. The rotation of the 

 telescope and of the analyser could be measured almost to a 

 minute by the aid of a system of two concentric circles, of which 

 tin: first, fixed to the support of the apparatus, was divided into 

 degrees and thirds of a degree; and the other, moveable with 

 the telescope, carried two opposed verniers giving the minute; 



