488 M. A. Benin on Circular Magiietk Polarization. 



Wishing to assure myself that in the electro- magnet of M. 

 Becquerel, the axis of which is filled up, the rotation on the 

 pole occurred in the same manner as in the hollow reels where 

 I had observed it, I endeavoured to receive the ray in the axis 

 of the reel by the aid of reflexion. For this purpose I fixed 

 Norremberg's apparatus on the pole. The tinned horizontal 

 mirror being placed directly on the end face of the electro- 

 magnet, I received on the oblique glass the light of the clouds. 

 This light is, as is known, reflected a first time fi'om above 

 downwards upon this glass ; then a second time on the hori- 

 zontal glass, which sends it vertically from below upwards to 

 the analyser ; only, from its being very imperfectly polarized, 

 it is difficult to determine the plane of polarization, and con- 

 sequently the rotation which it might experience. But this 

 becomes easy by placing upon the crystal bearer the double 

 rotating plate of M. Soleil. The position of the plane of po- 

 larization is then determined by that of the analysing prism, 

 which gives the equality of tints in the two halves of the plate. 

 With this arrangement, let us place upon the horizontal mirror 

 the flint-glass of Prof. Faraday. As long as the current does 

 not pass, we observe no change, except that which arises from 

 a slight unannealed condition of the glass ; but as soon as the 

 current passes, we see the double rotation plate vary its tints 

 in an extremely brilliant manner ; and to re-establish the iden- 

 tity, it will be necessary to turn the analyser ten degrees if the 

 flint-glass is 18 millimetres in thickness, and twenty-one de- 

 grees if it is 48. As to the direction of the rotation, it takes 

 place from right to left when the pole is southern, and from 

 left to right when it is the contrary. 



This method allows of our observing the action of an elec- 

 tro-magnetic reel parallel to its axis, in another direction than 

 this axis, and the results thus obtained deserve attention. 



Let us picture to ourselves the horizontal section of the 

 electro-magnet of M. Becquerel. It is composed of two equal 

 circles corresponding to the two vertical arms, not exactly 

 touching, but only 1 centimetre apart and 23 centimetres in 

 diameter. Each of these circles is formed by an interior 

 circle of 11 centimetres, which is the section of the iron nu- 

 cleus, surrounded by a copper ring of 6 centimetres in width, 

 appertaining to the reel properly so called. If the flint-glass is 

 moved along the line of the centres whilst the electro-magnet 

 is in action, the following takes place. In the middle, equi- 

 distant from the centres, the rotation will be null; it will in- 

 crease until in contact with the iron, where it will be 9 de- 

 grees; then quite close to it, on the iron nucleus, it will in- 

 crease suddenly to 21 degrees. It will remain nearly fixed 



