Footnote 6: Professor Stokes and Mr. Vernon Harcourt some time ago made experiments with phosphatic glass, and some of this material was worked into a lens by Mr. Grubb, who states that “the result was successful so far as the obtaining of specimens of phosphatic glass with rational spectra; but phosphatic glass is almost unworkable, and when the experiment was tried on a siliceous glass it failed. Some alleviation of this secondary spectrum can be got by using a triple objective, but with, of course, a corresponding loss of light.” Footnote 7: Mr. Browning’s method of silvering glass specula is as follows:— Prepare three standard solutions: Solution A { Crystals of nitrate of silver 90 grains } Dissolve. { Distilled water 4 ounces } Solution B { Potassa, pure by alcohol 1 ounce } Dissolve. { Distilled water 25 ounces } Solution C { Milk-sugar (in powder) ½ ounce } Dissolve. { Distilled water 5 ounces } Solutions A and B will keep, in stoppered bottles, for any length of time; Solution C must be fresh. To prepare sufficient for silvering an 8 in. speculum, pour two ounces of Solution A into a glass vessel capable of holding thirty-five fluid ounces. Add, drop by drop, stirring all the time (with a glass rod), as much liquid ammonia as is just necessary to obtain a clear solution of the grey precipitate first thrown down. Add four ounces of Solution B. The brown-black precipitate formed must be _just_ re-dissolved by the addition of more ammonia, as before. Add distilled water until the bulk reaches fifteen ounces, and add, drop by drop, some of Solution A, until a grey precipitate, which does not re-dissolve after stirring for three minutes, is obtained; then add fifteen ounces more of distilled water. Set this solution aside to settle; do not filter. When all is ready for immersing the mirror, add to the silvering solution two ounces of Solution C, and stir gently and thoroughly. Solution C may be filtered. The mirror should be suspended face downwards about ½-inch deep in the liquid, by strings attached to pieces of wood fastened to the back of the mirror with pitch, and before being immersed should be cleaned with nitric acid and washed with distilled water. The silvering is completed in about an hour, and when finished the surface should be washed in distilled water and dried, and then polished with soft leather, finishing with a little rouge. The following method is used by M. Martin:— Make solutions: 1. Nitrate of silver 4 per cent. 2. Nitrate of ammonia 6 per cent. } perfectly free 3. Caustic potash 10 per cent. } from carbonates.