482 M. Verclet on the Optical Properties developed in 



and on the quantities of free magnetism which are there accu- 

 mulated ; and the law to be determined is that which would be 

 observed if the magnetic action emanated from a single centre 

 whose distance and force could be made to vary. As, in order 

 to exhibit the phamoniena on a scale sufficiently large for obser- 

 vation, we are forced to place the transparent substance very 

 near the electro-magnet, we cannot assume that the action is 

 analogous to that of one or of two poles, according as the electro- 

 magnet has one or two branches. 



We are ignorant also, in consequence of the complicated form 

 of the apparatus, of the position of the poles in the electro-magnet. 

 To remove the difficulties which arise from the inequality of the 

 optical action of different plates of the same transparent body, I 

 thought it would be sufficient to use an artifice employed by 

 several authors, especially Mr. Faraday and M. Plvicker, in the 

 study of diamagnetic attractions and repulsions. 



If we adapt to the two extremities of an electro-magnet two 

 large armatures of soft iron, which present, opposite each other, 

 two wide vertical faces, the space comprised between these ver- 

 tical faces becomes, in Faraday's words, a magnetic field of equal 

 intensity; i. e. a molecule of magnetic fluid* placed anywhere 

 in this space, excepting in the vicinity of its extremities, is sub- 

 mitted to a system of actions whose resultant varies very little in 

 magnitude and direction. In fact, the two terminal surfaces of 

 the armatures are charged with the greatest part of the free 

 magnetic fluids (as experiment shows), and these fluids are 

 distributed almost uniformly, although with a tendency to accu- 

 mulate towards the edges. 



If we now conceive a magnetic molecule placed iu the centre 

 of this space, it would be submitted to a system of forces whose 

 resultant Avould have a certain magnitude, and whose direction 

 would be, on account of the symmetry, the right- line which 

 unites the centres of the two terminal surfaces, i. e. the right line 

 generally called the line of the poles. If we conceive the mag- 

 netic molecule to deviate from this position by receding from one 

 of the armatures, it would approach the other; by approaching 

 the edge of one armature it would recede from the opposite edge. 

 Consequently, according as certain actions became more intense 

 or more inclined towards the line of the poles, others would 

 become less intense or less inclined. We can conceive, then, 

 that in a certain region there could be established a compensa- 

 tion almost complete, in such a manner that, as before men- 



* It is scarcely necessary to say, that when I employ the expressions 

 magnetic fluid and other similar ones, it is only for the sake of representing 

 the phenomena more conveniently, and without committing myself to any 

 particular theory of magnetism. 



