328 M. Pouillet on the Recent Researches of Prof. Faraday. 



pose, for example, that the one on the right has turned blue ; 

 if the current is passed in a contrary direction, it is that on 

 the left which this time turns blue in the same manner. 

 Thus, by reversing the poles of the electro- magnet, the action 

 which it exercises on the flint-glass, or on the light which 

 traverses it, is also suddenly reversed. Here then we see the 

 action in question rendered evident in the most striking and 

 incontestable manner. 



In the circumstances of which I have just spoken, ten ele- 

 ments are more than sufficient to exhibit it to a practised eye ; 

 but with a hundred elements, it assumes such an intensity 

 that persons the most unaccustomed to these kind of observa- 

 tions could not fail to perceive it as a perfectly characterized 

 phaenomenon. 



Before seeking to ascertain whether this effect, at once so 

 novel and so extraordinary, results from a direct action of the 

 magnetic fluid upon light, or from an indirect action, in which 

 the ponderable matter of the flint-glass intervenes, or at least 

 the collective forces to which this matter is subjected in order 

 to exist in molecular equilibrium, it is necessary first to de- 

 termine precisely what is the nature of the effect produced, 

 and to seek above all to measure its intensity, in order to 

 ascertain what are the conditions under which the phaenome- 

 non is shown with the greatest energy. 



For this purpose, instead of observing directly the coloured 

 tints which the quartz gives by the lamp perpendicular to the 

 axis, it is necessary to recompose what M. Biot has called 

 the tint of passage. This is done by placing before the ob- 

 jective several systems of blue and greenish glasses ; but I 

 found in the cabinet of the Conservatoire some glasses very 

 slightly coloured blue, which give to this tint a sensitiveness 

 still greater than that obtainable by other means. When 

 these glasses are interposed in the pencil, the tints of the 

 quartz become of a light lilac, on which the least changes of 

 shade are appreciable ; the uncertainties which are presented 

 by the zero of the compensator disappear, and it becomes 

 possible not only to perceive, but to measure the effects which 

 correspond to thicknesses of quartz of a hundredth of a milli- 

 metre. 



The instrument thus modified, the compensator being at 

 zero, and the polarizing prisms of the object-glass and of the 

 ocular being suitably regulated in their relative positions, the 

 experiment may be proceeded with ; only there is one thing 

 which requires mention, not to pay attention to the yellow 

 image, but to look exclusively at the lilac image, the two 

 halves of which are then exactly of the same shade. 



