482 M. A. Bertin on Circular Magnetic Polarization. 



rent itself during a series of experiments would produce dif- 

 ferences far more considerable, which must be avoided by 

 comparing only the rotations observed at short intervals, and, 

 as it were, the one following the other. 



It has been observed, that for the success of these experi- 

 ments it is indispensable that the glasses experimented with 

 should be annealed ; but happily this is not the case, other- 

 wise these researches would be impossible, since the greater 

 part of them are unannealed, or soon become so. When 

 such a glass is suitably placed between two crossed Nichol's 

 prisms, one or several black lines appear in it, which serve as 

 index points. On viewing one of these lines, which may nearly 

 always be isolated in the field of vision, it is seen to disappear 

 when the current passes ; and it is again found, on turning 

 the analyser, absolutely like the black image of annealed 

 glass. In white light it experiences the same variations of 

 tints as this, and it is always easy to determine the azimuth in 

 which it presents the tint of passage. It is true that, in con- 

 sequence of the movement of the plane of polarization, it is 

 not the same black ray which reappears, but another slightly 

 different. However, the rotation is not influenced by this 

 circumstance, for I have ascertained that it is independent of 

 the black ray to which the eye is directed. 



The most happy modification which has been made in the 

 original apparatus of Prof. Faraday, has been to cause the ray 

 of light to pass, not merely near the line of the poles, but 

 through this line itself, by piercing the electro-magnet in this 

 direction. This condition is satisfied in M. E. Becquerel's 

 electro-magnet by the perforated terminations which he places 

 on the two poles*, and to them must be attributed the greater 

 part of the force of this apparatus. The following table leaves 

 no doubt on this point. 



Rotations observed with the Electro-magnet ofM. Becquerel. 



* Ann. de Chim, et de Phys., third series, vol. xvii. p. 437. 

 f I am indebted to M. Dumas for this flint-glass. 



