TRANSACTIONS OF THE SECTIONS. 55 



by MM. Fizeau and Foucault, which consists in decomposing the white light after 

 it has traversed the apparatus in which it has suffered a certain modification ; and 

 in examining how this modification varies from one extremity to the other of the 

 spectrum thus obtained, selecting especially, for the numerical measure of the rela- 

 tive effect, the seven principal rays which Fraunhofer has defined, and the length 

 of those undulations he has determined. As the exact determination of the position 

 of a plane of polarization requires that the light shall have a certain intensity, I have 

 been obliged to confine myself to measuring the relations of beams which correspond 

 with five rays, C, D, E, F and G. 



As in my previous researches, in order to operate upon certain bodies well de- 

 fined and easily reproduced, I have always experimented upon liquids, contained in 

 tubes closed at their extremities by transparent plates ; placing these tubes in the 

 interior of a strong electro-magnetic coil, and so arranged that their two ends shall 

 sufficiently pass the edges of the coil, to obviate the necessity of taking into account 

 the action which the transparent plates themselves might exercise on polarized light. 

 But under these conditions the employment of a powerful current was rendered ab- 

 solutely necessary by the feebleness of the phenomena ; and the nature of the experi- 

 ments requiring that each liquid should remain for a long time under observation, 

 an elevation of temperature was produced which easily reached 50° or 60° Centigrade, 

 and which produced a contraction of the rotations observed, which it was very diffi- 

 cult to correct. 



The only way of avoiding this source of error, was to place the tube containing the 

 liquid in an annular collar continually traversed by a stream of cold water within 

 the electro-magnetic coil, — a considerable complication to the apparatus. 



The coil which I employed was not less than 45 centimetres in length, 15 centi- 

 metres in internal, and 30 centimetres in external diameter. It contained more than 

 80 kilogrammes of copper wire 225 millimetres in diameter, and was set in action 

 by a Bunsen's battery of twenty or thirty elements. 



I have not yet quite finished my experiments, but I am now in a position to esta- 

 blish one result, which does not appear to me to be unworthy of being communi- 

 cated to the Association. M. Wiedemann, in a note published in 1851, believed that 

 he might deduce from a small series of experiments, that if one submitted to the 

 action of magnetism a substance capable of itself of turning the plane of polarization, 

 such as the spirit of turpentine or essential oil of lemon, the rotation proper to the 

 substance and the magnetic rotation were proportional to one another through all 

 the colours of the spectrum. In order to submit to a decisive proof this law, which 

 would be, were it true, of immense theoretical importance, I have just examined an 

 extreme case, that of tartaric acid. We know that solutions of this acid induce in 

 the planes of polarization rotations which do not increase from the red to the violet, 

 as in ordinary cases, but which present in the interior of the spectrum a maximum 

 whose exact position varies with the strength of the solution. Were the relations 

 admitted by M. Wiedemann correct, the magnetic rotations of tartaric acid should 

 present the same anomaly. My experiments have, however, proved, on the contrary, 

 that the magnetic rotations in different solutions of this acid always increase from 

 the red to the violet. There is no essential relation between the two orders of phe- 

 nomena, as there is no analogy between their causes. 



On the other hand, my experiments show how the two phenomena may have ap- 

 peared in some cases proportional. They show, in fact, that in all cases the mag- 

 netic rotations of the plane of polarization increase very rapidly from the red to the 

 violet, but that the product of the rotations by the square of the length of the un- 

 dulations, increase very slowly between the same limits ; and one recognizes in this 

 announcement, that which experiment has long ago demonstrated in most of the 

 natural rotatory powers. 



Results of Self-registering Hygrometers. By E. Vivian, M.A., Torquay. 



Mr. Vivian reported to this Section a series of observations made with his new 



self-registering hygrometers, which were first exhibited before the Association at its 



Cheltenham meeting. One is a combinalion of the ordinary wet and dry bulb and 



the differential thermometers, registering the maximum and minimum, or range during 



