Having occasion to require hitherto unapproached lightness and optical accuracy in such mirrors, I got my assistant to try making them of fused quartz, slices being cut by a diamond wheel from a rod of that material. Chips of natural quartz were also obtained from broken "pebble" spectacles, and these were worked at the same time. The resulting mirrors were certainly superior to the best we could make from glass, but the labour of grinding was greater, and the labour of polishing less, than in the latter case. The pebble fragments gave practically as good mirrors as the fused slices. For the future it will be better always to make galvanometer mirrors from quartz crystals. These may be easily sliced, as will be described in Sec. 74. The slices are dressed on a grindstone according to instructions already given for small lenses. The silvering of these mirrors is a point of great importance. After trying nearly every formula published, we have settled down to the following. A solution of pure crystallised nitrate of silver in distilled water is made up to a strength of 125 grams of the salt per litre. This forms the stock solution and is kept in a dark bottle. Let the volume of silvering liquor required in any operation be denoted by 4 v. The liquor is prepared as follows: I. Measure out a volume v of the stock solution of silver nitrate, and calculate the weight of salt which it contains; let this be w. In another vessel dissolve pure Rochelle salt to the amount of 2.6 w, and make up the solution to the volume v. These two solutions are to be mixed together at a temperature of 55 deg. C, the vessels with their contents being heated to this temperature on the water bath. After mixing the liquids the temperature is to be kept approximately constant for five minutes, after which the liquor may be cooled. The white precipitate which first forms will become gray or black and very dense as the liquid cools. If it does not, the liquor must be reheated to 55 deg. C, and kept at that temperature for a few minutes and then again allowed to cool. The solution is in good order when all the precipitate is dense and gray or black and the liquor clear. The blacker and denser the precipitate the better is the solution. The liquor is decanted and filtered from the precipitate and brought up to the volume 2 v by addition of some of the wash water. II. Measure out a volume 0.118 v of the stock solution into a separate vessel, and add to it a 5 per cent solution of ammonium hydrate, with proper precautions, so that the precipitate at first formed is all but redissolved after vigorous shaking. It is very important that this condition should be exactly attained. Therefore add the latter part of the ammonia very carefully. Make up the volume to 2 v.