Through the Looking-Glass, and What Maxwell Found There

Abstract

In his essay on “Mechanical Explanation at the End of the Nineteenth Century,” Martin Klein remarked on “the complexity and variety of the ideas that were current then”: this was “a time of probing and testing.”¹ These judgements are aptly descriptive of the physics of James Clerk Maxwell, and especially of his most famous innovation, the electromagnetic theory of light. His statement in 1862, that “light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena,”² implied the unification of optics and electromagnetism in terms of a mechanical theory of the ether that had both optical and electromagnetic correlates.³ When he wrote his seminal Treatise on Electricity and Magnetism (1873) it might have been anticipated that Maxwell would broaden the scope of his electromagnetic theory of light to encompass an electromagnetic theory of the reflection and refraction of light. But he did not do so; and though he gave a detailed treatment of the Faraday magneto-optic rotation (where he appealed to the rotation of molecular vortices in the ether), the range of his optical theory remained essentially similar in its physical content to that first advanced in 1862 and subsequently amplified in a major paper published in 1865.

This essay draws on work carried out in the preparation of my edition of The Scientific Letters and Papers of James Clerk Maxwell, which is in course of publication by Cambridge University Press. I am grateful to the Syndics of the Cambridge University Library and to the Council of the Royal Society for kind permission to reproduce documents. I am grateful to the Council of the Royal Society and the National Science Foundation for generous financial support of my work on the edition; and to the Department of the History of Science, Harvard University for providing facilities for this work. I thank Jed Buchwald and Alan Shapiro for discussion of the theme of the paper at a conference sponsored by the Dibner Institute and organized by A.I. Sabra, to whom I am grateful for the invitation.