"If an error of concavity presents itself the process of polishing 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 one 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 Professor Langley has informed me that his assistant, Mr. Keeler (J. E.), has seen the nickel line clearly between the D lines. This speaks for the superiority of the surfaces over those polished on broadcloth. "In polishing prisms I prefer to work them on top of the polisher, 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 results are so good that it well repays the few hours' work it requires to master the few difficulties it presents." Fig. 58. Sec. 70. Casting Specula for Mirrors. According to Sir H. Grubb (loc. cit.) the best alloy is made of four atoms of copper and one of tin; this gives by weight, copper 252, tin 117.8. The copper is melted first in a plumbago crucible; the tin is added gradually. Of course, in the process of melting, even though a little fine charcoal be sprinkled over the copper, some loss of that metal will occur from oxidation. It is convenient in practice, therefore to reserve a portion of the tin and test the contents of the crucible by lifting a little of the alloy out and examining it. The following indications may be noted: When the copper is in excess the tint of the alloy is slightly red, and the structure, as shown at a fractured surface, is coarsely crystalline. As the proper proportions are more nearly attained, the crystalline structure becomes finer, the colour whiter, and the crystals brighter. The alloy is ready for use when the maximum brightness is attained and the grain is fine.