M. Pouillet on the Recent Researches of Prof. Faraday. 329 



As soon as the current passes, one of the halves of this 

 image, for instance that on the right, is seen to turn blue ; we 

 observe that this tint is persistent as the current itself, and 

 we may be convinced that, from the first instant, it acquired 

 its whole value, that is to say, that the prolonged duration of 

 the action adds nothing perceptible to it. The compensator is 

 then moved in the proper direction ; the difference of the tints 

 gradually disappears in proportion as it advances, and with a 

 little practice the point at which the equality is re-established 

 is soon found. The number of divisions is noted down, and 

 we thus obtain a measure, or at least an approximate measure 

 of the effect produced, — say twenty divisions. 



When subsequently the current is passed in a contrary di- 

 rection, it is the other half of the tint, that to the left, which 

 turns blue, and it is in the other direction that the compen- 

 sator has to be moved to re-establish the equality. No inter- 

 val of time is appreciable between the change of the current 

 and the change of effect upon the light, and it is again instan- 

 taneously that the tint takes all its value. When the optical ap- 

 paratus is well-adjusted, and the electrical communications are 

 equally good in both directions, the ground gone over by the 

 compensator is the same in the two cases, that is to say, that 

 if it progressed in the first twenty divisions to the right, it 

 should in the second proceed twenty degrees to the left. 



These opposite effects and the corresponding measures may 

 be repeated indefinitely, either with the same or a different 

 number of pairs of the battery ; and a few hours are sufficient, 

 during which the action of the battery is nearly constant, to 

 pass in review a great number of diaphanous substances, and 

 to obtain a first approximation on the relative sensitiveness 

 with which they obey the magnetic influence. 



When the substances submitted to the test are more or less 

 coloured, it is necessary to vary the systems of glasses intended 

 to produce the tint of passage, and we do not always succeed 

 in composing a tint equally delicate and easy of observation. 

 It might happen consequently that some substances, even 

 slightly coloured, when submitted to these modes of observa- 

 tion, would appear much less energetic than they are in 

 reality. 



Let us pause then at the diaphanous substances, and ob- 

 serve that in the experiment with the flint-glass cited above, 

 it was necessary to advance the compensator twenty divisions 

 to the right and twenty divisions to the left, according as the 

 current passed in one direction or the other. Let us bear in 

 mind, that if, instead of interposing on the passage of the 

 pencil a prism of flint-glass submitted to the electro-magnet, 



