CLEANING LENSES It is easy to damage a lens by improper cleaning, optical glass being generally much softer than other kinds. Apart from actual scratches the surface is liable to become dulled, a condition that affects the " rapidity." The necessity for frequent cleaning is obviated by fitting all lenses with caps to both front and back combinations, or by keeping them in air tight cases. When a lens requires cleaning it should first be dusted with a camel-hair brush or tuft of cotton-wool, and then carefully wiped with a very soft silk or linen handkerchief, or with a soft wash-leather. If the surface still appears cloudy, a single drop of pure alcohol should be put on each of the surfaces, which should be carefully wiped until quite dry. The spirit must not be allowed to run between the lens and its brass cell, and care must be taken not to remove the dead black coating from the lens cell. The edges of the lens in contact with the
cell are best cleaned with the pointed end of a bit of soft wood over which the rag is stretched. Dr. Miethe recommends the use of pith, such as that of the rush or elder, for this purpose. A lens that has become scratched or dulled should be sent to the maker to be repolished. The greatest mistake an amateur can make is to attempt to repolish ,a lens with rouge, putty powder, etc. ; even the maker cannot repolish a badly scratched lens so that it will work quite as well as when it was new, therefore the result of unskilled work can easily be anticipated. A lens should not be wetted if it can be avoided, and in the case of condensation of steam or dew upon the surfaces, the moisture should be re moved as quickly as possible.