In addition to possessing the requisite optical constants, a good colour and perfect homogeneity, certain other properties are essential in good optical glass. These are the general physical and chemical qualities which are essential in all good glass, but especially emphasised by the fact that the requirements for optical glass are more stringent than for any other variety of the material. Thus chemical stability is of the greatest importance, for the best lenses would soon become useless if the action of atmospheric moisture were to affect them appreciably--the polished surfaces would rapidly become dull and the whole lens would soon be rendered useless. The conditions governing the chemical stability of glass and the methods of testing this quality have already been indicated (Chapters I. and II.). The harder varieties of optical glass, such as the glasses quoted in the above table under the names of “Hard Crown” and Boro-Silicate Crown, are probably among the most durable and chemically resistant of all varieties of glass, but as we have already indicated, when extreme optical properties are required, the necessary chemical composition of the glass always entails a sacrifice of this great chemical stability, until a limit is reached where valuable optical properties no longer counterbalance the serious disadvantage of a chemical composition which renders the glass liable to rapid disintegration. In certain special cases it is, perhaps, possible to protect lenses made of such unstable glass by covering them with cemented-on lenses of stable glass, but this device entails concomitant limitations in the design of the optical system and is, therefore, rarely used. In any case, however, it is well for the lens-designer to consider the relative stability of the glasses employed when arranging the order in which they are to be used, since it is obviously preferable to put a hard, durable glass on the outside of his system, where it is most directly exposed to atmospheric moisture, and is also subject to handling and “cleaning” by inexpert hands. This latter factor is a very important one for the life of any lens. In the first place, a glass surface is very seriously affected by the minute film of organic matter which is left upon it when it has been touched with even a clean finger; unless the glass is of the best quality in this respect, such fingermarks readily develop into iridescent spots and may even turn into black stains. Particles of dust allowed to settle on the surface of the glass will affect it in the same way, so that the protection afforded by mere mechanical enclosure in the tube of an instrument is of decided value in preserving a glass surface. It should, however, be noted that in some instances the interior metal surfaces of optical instruments are varnished with substances that give off vapours for a long time after the instrument is completed, and in that case the inside lenses are apt to be tarnished in consequence.