(_i._) 1832. Elementary guesses by Brewster and Prinsep, the former of whom ascribed the phenomenon to some supposed molecular changes in the metal at the surface, due to a pattern having been stamped on the front to imitate that on the back, and then ground off. The latter ascribed it more correctly to differences of curvature of surface, but also fell into the error of supposing the ornament to have been stamped. (_ii._) 1844-1853. French investigations by Arago, Julien, Person, and Maillard. Of these investigators Person gave the suggestion of the true cause, namely, minute differences of curvature in the polished surface, a circumstance which he proved by covering the mirror with a piece of paper having a circular hole, about one centimetre in diameter, and which, when moved about over the mirror (in sunlight), caused as a reflected image a spot of light, the size of which varied from point to point of the mirror. He also soldered a narrow slip of sheet metal behind a polished daguerreotype plate, and found that when the latter was slightly bent the image from the front showed a luminous line at the corresponding place. Maillard later adopted Person’s theory, and confirmed it by employing an optician to burnish on the lathe a piece of metal which had raised markings on its back. He also noted that a scratch on the mirror back gives a bright line in the image. (_iii._) 1864-66. Investigations of Govi, in Italy, and his controversy with Brewster thereon. Govi frankly adopted the views of Person, and confirmed them by an experiment based on the mode adopted by the constructors of reflecting telescopes for testing the accuracy of the figure given by the grinding machinery to specula. He caused a magic mirror to reflect upon a screen the image of a fine grating of lines, ruled upon glass with a diamond, and placed close to a brilliant point of light. By the distortions which the mirror produced in these lines he found the whole reflecting surface to be minutely undulated over with slight variations of curvature in complete correspondence with the raised arabesques on the back. These undulations, he observed, escape notice when one is looking into the face of the mirror because they are so gentle, and are such that, to perceive them, we require organs that are more delicate than our own. On the publication of this account, and of a translation of it in the “Scientific Review” for 1865, Sir David Brewster wrote to say how, in 1832, he had explained the phenomenon as being due to differences of density or other quality of molecular structure, or to fine scratches produced by trickery, and doubting Govi’s explanation. He declared that similar phenomena had been produced by stamping patterns into the face of brass and grinding them down to be invisible on inspection. He considered the only way in which they could be proved to be due to delicate differences of curvature was either to show that they disappeared on re-surfacing the mirror with