The cells of the pith (Pl. IX. fig. 7) also present interesting varieties, being sometimes arranged in a single row, at others in two or more rows (fig. 6). These are best seen in hairs recently immersed in spirit or in oil of turpentine; for if the hair be too long soaked in these liquids, the air will be entirely displaced by them. The cells of the pith appear black by transmitted and white by reflected light, in the dry hairs, from the presence of air. They may be well examined in the hair of the mouse (figs. 6 & 7), or in that of the mole. Wool, which is the hair of the sheep, consists of curled fibres (Pl. IX. fig. 10), in which the imbricated arrangement of the surface-scales is very distinctly seen. In Pl. IX. figs. 10-13 the fibres of wool, flax or woody fibre, cotton, and silk are represented together, to allow of comparison; for the microscope is of great assistance in discriminating these substances when existing in textile fabrics. The fibres of wool (fig. 10) are distinguished by their solidity, wavyness, and the imbricated scales; those of flax (fig. 11) by their thick walls, great length, acute ends, and their knotty appearance at intervals. The fibres of cotton (fig. 12) are soft, flaccid, flattened, and often twisted; and those of silk (fig. 13) are solid and very slender. By a little chemical testing, the discrimination is made still more easy; but for an account of this I must refer to the Dictionary. BIRDS.--In the Class of Birds, the structure of the _feathers_ deserves special notice. Feathers are epidermic formations, or consist of aggregations of epidermic cells, yet so altered by compression and fusion together that the cell-structure is in most parts difficult to detect. In a feather three parts are distinguishable,--the transparent cylindrical quill; its opake continuation, which is more or less flattened at the sides, forming the shaft; and the vanes or beards, which arise from the sides of the shaft, consisting of numerous closely set, parallel, flattened fibres, called the barbs. The structure of the barbs forms the interesting object to the microscopist. On examining a piece of the coloured vane of a somewhat large feather (Pl. IX. fig. 14), a row of fine parallel colourless filaments (_pinnæ_) will be observed, arising from the opposite sides, the filaments of one side lying obliquely across those arising from the other; and while the filaments or pinnæ of one side present a row of little teeth (fig. 14 _c_) near their base, those of the opposite side (fig. 14 _b_) are provided with as many hooks near their apex, which curve over the teeth to connect the barbs together. This curious arrangement is adapted to keep the parts of the feather firmly united, and yet to allow of their play and flexibility. To observe this structure, a portion of a vane should be soaked in oil of turpentine, and mounted in balsam.