Before anything else is done the edge of the objective should be marked with a hard lead pencil on the edge of both the component lenses, making two well defined v’s with their points touching. Also, if, as usual, there are three small separators between the edges of the flint and crown lenses, mark the position of each of these 1, 2, 3, with the same pencil. Forming another convenient pad of something soft, lift off the upper lens, take out the three separators and lay them in order on a sheet of paper without turning them upside down. Mark alongside each, the serial number denoting its position. Then when these spacers, if in good condition, are put back, they will go back in the same place rightside up, and the objective itself will go back into place unchanged. Now have at hand a wooden or fibre tub or basin which has been thoroughly washed out with soap and water and wiped dry. Half fill it with water slightly lukewarm and with a good mild toilet soap, shaving soap for example, with clean hands and very soft clean cloth, go at one of the lenses and give it a thorough washing. After this it should be rinsed very thoroughly and wiped dry. As to material for wiping, the main thing is that it must be soft and free from dust that will scratch. Old handkerchiefs serve a good turn. Dr. Brashear years ago in describing this process recommended cheese cloth. The present day material that goes under this name is far from being as soft at the start as it ought to be, and only the best quality of it should be used, and then only after very thorough soaking, rinsing and drying. The very soft towels used for cleaning cut glass, if washed thoroughly clean and kept free from dust, answer perfectly well. The cheese cloth has the advantage of being comparatively cheap so that it can be thrown away after use. Whatever the cloth, it should be kept, after thorough washing and drying, in a closed jar. Rinsing the lens thoroughly and wiping it clean and dry is the main second stage of cleansing. It sometimes will be found to be badly spotted in a way which this washing will not remove. Sometimes the spotting will yield to alcohol carefully rubbed on with soft absorbent cotton or a bunch of lens paper. If alcohol fails the condition of the surface is such as to justify trying more strenuous means. Nitric acid of moderate strength rubbed on with a swab of absorbent cotton will sometimes clear up the spotting. If this treatment be used it should be followed up with a 10 per cent solution of pure caustic potash or moderately strong c.p. ammonia and then by very thorough rinsing. Glass will stand without risk cautious application of both acid and alkali, but the former better than the latter.