The Sidereal Messenger Vol. 5, No. 5 May 1886 pg. 149 A Practical Method of Working Rock Salt Surfaces for Optical Purposes.* Jno. A. Brashear, Pitstburg. Two years or more since Prof. LANGLEY asked me to under- take the work of polishing the rock salt trains so frequently called into use in his well-known researches in obscure heat rays. These surfaces, under the very best conditions, are ephem- eral in their character, owing to the deliquescent nature of the material, and in order to get the best results from them, fre- quent repolishing and refiguring are absolutely necessary. I have been informed that the French opticians polish all rock salt surfaces upon broadcloth; and indeed, almost all surfaces I have tested show them to have been finished upon some yield- ing material, as the edges are almost always rounded, or as I would call it, over corrected. This is fatal to good results in any optical surface. Mr. GEORGE CLARK of ALVAN CLARK & SONS polished a prism for Prof. LANGLEY which turned out to be beautiful in polish and figure. His method was to use a pitch polisher with "diamantine" (a fine variety of Vienna lime) as the polishing material. A strong brine was used in- stead of water, in the ordinary way. Mr. CLARK informed me that the one great difficulty he met with was to wipe the prism surfaces after the polishing was completed, he using the arm or palm of the hand, in preference to anything else. On a few occasions I have succeeded in this way, but where success may be had once, failure may result twenty times, for if any mois- ture is left on the surface of the prism or lens, even for a mo- ment of time, it is ruined. Happily I have no trouble in this respect now, and as my method is easily carried out by any physicist who desires to work with rock salt surfaces, it gives me pleasure to explain it. For polishing a prism I make an ordinary pitch bed of about *Read before the American Association for the Advancement of Science, Ann Arbor Meeting, August, 1885. pg. 150 two and one-half or three times the area of the surface of the prism to be polished. While the pitch is still warm I press up- on it any approximately flat surface, such as a piece of ordinary plate glass. The pitch bed is then cooled by a stream of water, and conical holes are then drilled in the pitch with on ordinary counter sink bit, say 1/4 in. diameter, and at intervals of half an inch over the entire surface. This is done to relieve atmos- pheric pressure in the final work. The upper surface of the pitch is now very slightly warmed and a true plane surface (us- ually a glass one, prepared by grinding and polishing three sur- faces in the ordinary way), previously wetted is pressed upoon it until the pitch surface becomes an approximately true plane it- self. Fortunately moderately hard pitch retains its figure quite persistently through short periods and small changes of temp- erature, and it always pays to spend a little time in the prepa- ration of the pitch bed. The polisher being now ready, a very small quantity of rouge and water is taken upon a fine sponge and equally distributed over its surface. The previously ground and fined salt surface (this work is done the same as in glass working) is now placed upon the polisher and motion instantly set up in diametral strokes. I usually walk around the polisher while working a surface. It is well to note that the motion must be constant, for a moment's rest is fatal to good results, for the reason that the surface is quickly eaten away, and irregularly so, owing to the holes that are in the pitch bed. Now comes the most impor- tant part of this method. After a few minutes' work the moisture will begin to evaporate quite rapidly. No new appli- cation of water is to be made, but a careful watch must be kept upon the pitch bed and as the last vestige of moisture disap- pears the prism is to be slipped off the polisher in a perfectly horizontal direction, and if the work has been well done, a clean, bright and dry surface is the result. The surface is now tested by the well known method of interference from a perfect glass test plate. pg. 151 If an error of concavity presents itself the process of polish- ing is gone over again, using short diametral strokes. If the error is one of convexity, the polishing strokes are to be made along the chords, extending over the edge of the polisher. The essential feature of this method is the fact that the surface is wiped dry in the final strokes, thus getting rid of the one great difficulty of pitch polishing, a method undoubtedly far superior to that of polishing on broadcloth. If in the final strokes the surface is not quite cleaned I usually breathe upon the pitch bed, and thus by condensation place enough moisture upon it to give a few more strokes, finishing just the same as before. In ten minutes I have polished prisms of rock salt in this manner that have not only shown the D line double, but Prof. LANGLEY has informed me that his assistant, Mr. KEELER (J. E.), has seen the nickel line clearly between the D lines, as well as every line that can be seen by the use of a good flint glass prism, al- the dispersion is not so great. This speaks for the su- periority of the surfaces over those polished on broadcloth. In polishing prisms I prefer to work them on top of the pol- isher as they can be easily held, but as it is difficult to hold lenses or planes in this way, without injuring the surfaces, I usually support them in a block of soft wood, turned so as to touch only at their edges, and work the polisher over them. Though it takes considerable practice to succeed at first, the re- sults are so good, that it well repays the few hours' work it re- quires to master the few difficulties it presents.