M. Pouillet on the Recent Researches q/'Prof. Faraday. 327 



The instrument of M. Soleil, the construction of which I 

 have just described, must be separated into two parts for the 

 experiments under consideration. The objective and the 

 ocular parts were mounted separately on my frame of diffrac- 

 tion*, which is most readily adapted for all the researches in 

 which it is required to centre the apparatus on the same 

 axis. A common lamp is placed before the objective part, 

 and a strong magnifier gives a pencil of light closely parallel, 

 which being propagated in the direction of the common axis, 

 traverses successively the object-glass, the pieces subjected to 

 the test, and the ocular; the distance between the objective 

 and the ocular may vary between tolerably distant limits, for 

 it may extend to nearly two metres, or only to a few centi- 

 metres, according to the nature of the observations. 



It is "important to remark that the pencil of light is always 

 horizontal, and the apparatus was accidentally arranged so 

 that the light was propagated from south to north, which may 

 assist us to define more easily the relative positions of the 

 polarized ray, of the electro-magnets, and of the bodies on 

 which they act. 



The electro-magnet is horizontal, that is to say, the plane 

 of the axes of its two branches is horizontal, and precisely at 

 the height of the pencil of light which traverses the appara- 

 tus ; moreover, the vertical plane, formed by the extremities 

 of the two branches or by the poles of the electro-magnet, is 

 parallel to this pencil, and may approach it more or less. 

 This being settled, if it be desired to submit to experiment, 

 for example, a parallelopipedon of flint-glass of ten or twelve 

 centimetres in length and terminated perpendicularly to its 

 length by two parallel planes, this parallelopipedon is first 

 arranged so that the ray polarized by the objective traverses 

 it in the direction of its axis, and if the flint-glass is pure and 

 well-annealed, as it must be for the success of the experiment, 

 its interposition produces neither deviation nor coloration in 

 the ray of light. The electro-magnet is then approached, ar- 

 ranging it in the same manner as if the piece of flint-glass were 

 a piece of iron to close it, and there is even no inconvenience 

 in arranging it so that the two poles of the electro-magnet are 

 in contact with the flint-glass ; the middle of the length of the 

 latter corresponds consequently to the interval which exists 

 between the two arms of the electro-magnet. 



When these arrangements have been made, a current is 



passed, and suddenly it is seen that the two tints of the red 



image, which correspond to the two opposed plates of the 



quartz of the object-glass, cease to be identical. Let us sup- 



* See my Elements de Physique, 4th edition, vol. ii. pi. 26. 



