The Sidereal Messenger Vol. 5, No. 7 July, 1886 pg. 222 NOTE ON REPOLISHING SURFACES OF ROCK-SALT -- In the Proceedings of the American Association for August 1885, Mr. J. A. BRASHEAR describes a method for producing optical sur- faces of rock-salt which in his hands gives results leaving little to be desired in the way of accuracy of figure and brilliancy of polish. These surfaces, however, when exposed to the air, de- teriorate more or less rapidly, according to the hygrometric conditions at the time, and soon become opaque and unfit for use. As the change in an ordinary dry state of the atmosphere is a gradual one, it is often a rather nice question to decide when it has advanced to such a point as to render the surfaces unfit for their work. The production of brilliant surfaces by Mr. BRASHEAR's method requires, besides the necessary appli- ances, much experience in such manipulations, doubtless well worth acquiring, but not always in the possession of the exper- imenter, and thus the restoration of the original polish when dimmed will in general be beyond his power. It very frequently happens, however, in investigations in radiant heat, where rock-salt finds its chief use in physical re- search, that the extreme accuracy of figure obtained by the above process is by no means necessary, whereas great trans- parency, i.e., with good material, brilliancy of polish of the surfaces is always absolutely essential. Under these condi- tions it is convenient to have some rough and ready way of renewing the polish of the dimmed surface, even at the ex- pense of its perfection of figure. After considerable experimenting with different substances, I have found nothing which gives better results for this pur- pose than thick, soft Canton flannel. It should be spread out on a smooth flat surface like a table top, or, better, a marble slab, with the furry side up. Breathe evenly over the surface of the rock-salt prism or plate and rub it quickly with circular and then with straight strokes upon the flannel. As soon as it glides easily, without much friction, remove it and examine pg. 223 the surface, and if not bright repeat the operation, using a different part of the cloth. It is best to wear a pair of kid gloves, to avoid the condensation of moisture from the hands. A surface almost as bright as the original can be produced in this way, and the irregularities of figure caused by the rubbing are surprisingly small. A prism by Mr. BRASHEAR, which had been treated as many as eight times in this manner by the writer, still defined the Frauenhofer lines with considerable sharpness, and the refracting angle had been altered less than 1'. The surfaces, when examined with a test plane by means of interference bands in sodium light, showed curious irregu- larities and a general slight convexity of figure. Lenses may be treated in the same manner, the cloth being held in the hand, unless a rounded surface of approximately the curvature of the lens is at command, but as they are usu- ally then they must be handled with great caution. It will of course be understood that nothing can be done in this way with surfaces originally bad, nor can much improvement be made in surfaces which have been badly corroded by exposure in too moist an atmosphere. The process applies only to sur- faces of good figure which have become dimmed by exposure under ordinary conditions. After being repolished in this way as many times as experience shows is allowable, the surfaces must be treated by a more perfect process like Mr. BRASHEAR'S in order to restore their original accuracy of figure. J. E. KEELER