John Frederick William Herschel to Faraday   9 November 1845

Collingwood Hawkhurst Kent | Nov 9, 1845

My dear Sir

I have this morning read with great delight a notice in the Athenaeum¹ of your experiments proving the connexion of Light with Magnetism[.] In the first place let me congratulate you cordially on a discovery of such moment which throws wide a portal into the most recondite arcana of nature[.] If I understand rightly the very meagre account given of your discovery it amounts to this - that the Electromagnetic current is capable of causing the plane of polarisation of a ray of light to revolve - for I can put no other probable interpretation on the expression "A beam of polarised light is deflected by the electric current so that it may be made to revolve between the poles of a magnet"[.]

If this be really the state of the case, it is what I have long anticipated as extremely likely indeed almost certain to be sooner or later experimentally demonstrated voici mes raisons[.]

There are 3 distinct classes of phaenomena in which a helicoidal dissymetry occurs - 1st the plagiedral faces on Crystals such as quartz which belong to an otherwise symmetrical system. These faces in some Crystals indicate a right handed in others a left handed dissymetry of the helicoidal kind. - 2dly the rotation of the plane of polarisation of a ray of light when transmitted through certain solids and liquids indicating a helicoidal dissymetry both in the ray and in the molecules or at least a capacity in the ray to be affected by that peculiarity in the latter. - 3dly in a rectilinear electric current which; [sic] deflecting a needle in a given direction as to right & left all around it indicates again a dissymetry of the same kind.

Now I reasoned thus[.] Here are 3 phaenomena agreeing in a very strange peculiarity. Probably this peculiarity is a connecting link physically speaking among them. Now in the case of the crystals & the light this probability has been turned into certainty by my own experiments. Therefore induction led me to conclude that a similar connexion exists and must turn up somehow or other between the electric current and polarised light, & that the plane of polarisation would be deflected by magneto electricity[.]

It is now a great many years ago that I tried to bring this to the test of experiment (I think it was between 1822 and 1825)² when on the occasion of a great magnetic display by Mr Pepys³ at the London Institution I came prepared with a copper helix in an earthen tube (as a non conductor) & a pair of black glass plates so arranged as that the 2d reflexion should extinguish a ray polarised by the first after traversing the axis of the copper helix[.]

I expected to see light take the place of darkness - perhaps coloured bands - when contact was made. The effect was nil. But the battery was exhausted and the wire long and not thick and it was doubtful whether the full charge remaining in the battery did pass, being only a single couple of large plates. -

There remained to be made another experiment before a negative could be considered as proved - vis to make the light move along a strait wire (or a combination of such[)] thus but this requiring preparation on the spot I could not then make and have never since had an opportunity, but the subject has often recurred to my mind and I have made frequent mention of it as a line of experiment worthy to be pursued.

You will be disposed to ask to what all this tends. Assuredly not to interfere for a moment with your claim to a beautiful discovery (for, though I may regret that I did not prosecute a train of enquiry which seemed so promising up to a decisive fact I consider it honour enough to have entertained a conception which your researches have converted into a reality) - but if it be not presumptuous in me to suggest a line of enquiry to you - I would willingly draw your attention to the other member of the triple coincidence above alluded to[.] There can be no doubt of the connexion of the Crystalline forces with magnetism & electricity. It seems to me now all but certain that the space or ether? surrounding an electrified wire or a magnet is in the same state with the space or ether intervening between the molecules of a plagi[h]edral Crystal. Polarised light is the test of that State - a helicoeidal, dissymmetrical State[.] This is the mode in which the phaenomena present themselves to my mind - not that light is electricity or magnetism - but that it is affected by them as by certain forms of matter; which therefore I conclude to be under the influence of magnetic currents in some concealed way circulating about them - and the line of enquiry I allude to is to ascertain whether crystals formed under the direct influence of magnetic currents or between the poles of magnets may not be thereby made to assume plagi[h]edral faces or shew other indications of assymmetrical action[.] If so the existence of the plagi[h]edral faces on Quartz is accounted for by the presence of such currents during their formation[.]

Believe me | my dear Sir | Yours most truly | J.F.W. Herschel