When light, in its way from a luminouß body to one's eye?, passes through more or less transparent media, it i-> more or less absorbed ; and the sp- ctroscopej which decomposes the light into the different kinds of coloured and nther rays of which it consists, is the aparatus which reveals what portion of the light is thua absorbed by the media through which it passes. This is the simple cause of the dark lines in the solar spectrum ; they are generated by the atmosphere which surrounds the sun, which atmosphere absorbs certain kinds of rays arid so forms the absorption bands, which we call the Frauenhofer lines, after the distinguished scientist Who first discovered them. The absorption bands (caused by transparent or translucent, colour or colourless, solid or liquid, substances) are of much practical importance, for every substance either absorbs different parts of the light or does it in a different manner ; so that the "absorption lines produced form characteristic properties of many bodies, by' which they may always be recognised. To make these' observations, it is best to use a white light giving a continuous spectrum, such as is produced by a gaa flame, a petroleum lamp, a white hot 'platinum wire, a hydro-oxygen lime light, 61? an electric light passing between carbon 1 points. As the solar spectrum has its own absorption bands, it is not to be preferred ; however it need not be totally rejected for this purpose, as the banda are very narrow and well defined, and the positions are well known, while the absorption bands of the solid and liquid transparent substances referred to are wide or broad, with somewhat undefined edges ; and therefore they are generally distinguished from the solar absorption lines: The substance to be examined is simply plaoed before the slit of the spectroscope, and the light passed through it. If this substance is, for instance, smoked glass (that is, a piece of glass coloured gray or black' by exposition to a smoky flame), it will be seen that the light of the epeotrum is dimmed over its whole extent, and this quite equally. The smoked glass, there fore, absorbs all kinds' of rays", all colours, in the same way, and is an example of a continuous or general asborption. Quite different, however, is it when we use coloured glass. 