William Whewell to Faraday   7 August 1846

Cliff Cottage, Lowestoft | Aug. 7, 1846

My dear Sir

I am looking forwards with great interest, as I suppose all persons who care for science are, to the continuation of your magnetical researches and speculations; and especially to those which refer to light. The establishment of a connection between magnetical and optical polarity appears to me a capital point, and the laws of this connection must have a wide bearing on our philosophy. I do not know if you have made any further examination of the amount of the deviation of the plane of polarization in your experiments (2152)¹ and especially of its amount for different colours. I believe one difficulty in the way of measuring this amount is the feebleness of the action which rotates the plane of polarization. It occurs to me that perhaps you may find some good suggestions as to the mode of making such experimental measures in Biot's researches on a similar subject, the circular polarization of fluids. When he analyzed the emergent ray by a double refracting prism he obtained the two images, neither of which vanished at any angle in consequence of the different amount of rotation of the plane for different colours. But he observed the extraordinary image when it became darkest, and noted the series of colours before and after this point, which are blue, very dark purple, red; and thus was able to measure the amount of rotation in different cases. His account of these experiments is in the Memoires of the Institut, Math et Phys. Tom XIII. p. 49 (1835)²[.] The relative rotation of different colours & other modes of experimenting must be used. You will see that Airy considers the determination of this point as very important³. I found after I had sent you some previous suggestions⁴ that they were of no value in consequence of the nature of your apparatus and the feebleness of the rotating power.

I am preparing for the press a new edition of my History and Philosophy⁵. If you have observed any mistakes in the former edition, I should be much obliged to your telling me of them.

I am always, my dear Sir, | Yours most truly | W. Whewell