The scales of various fish have been before mentioned as mounted “dry;” when, however, they are required for polarising objects they are generally mounted in balsam, and some few in liquid. The former method will be considered here. The eel affords a beautiful object for this purpose. The scales are covered by a thin “_skin_,” which may be slightly raised with a knife and then torn off, in the same manner as the covering of the geranium and other petals, described in Chapter II. The required portion may then be removed; or if a piece of skin can be procured as stripped off in cooking, the scales may be easily taken from the inner surface. They must then be washed and thoroughly cleaned. After drying, soak for a day in turpentine, and mount in the ordinary manner with balsam. This is a good polarising object; but the interest, and I think the beauty, is increased by procuring a piece of eel’s skin with the scales _in sitû_, washing and drying under pressure, and mounting in balsam as before. The arrangement of the scales produces beautiful “waves” of colour, which are quite soothing to the eye after examining some of the very gorgeous salts, &c. There are many scales of fish which are good subjects for the polariscope when mounted in balsam; but as they require no particular treatment, they need no mention by name. Among hairs we find some which are beautiful when mounted in balsam and examined by polarized light. Some, when wanted as common objects, are always used dry, as before mentioned; but if they are intended to be shown as _polarizing_ objects, they must be placed in some medium. The “Micrographic Dictionary” mentions a mode of making an interesting object by plaiting two series of white horse-hairs at an angle, mounting in balsam, and using with the polariscope. All hairs, however, must be steeped in turpentine for a short time before mounting, as they will thus be rendered cleaner and more transparent. When this is done, there is no difficulty in mounting them. Many of the “tongues” of fresh-water and marine mollusca are deeply interesting and most beautiful objects when examined by polarized light. As these are usually mounted in balsam, I mention them in this place; but as they must be removed from the animals by dissection, particulars respecting them will not be entered into until we come to the part in which that operation is described (Chapter V.).