paper. Serrated cells and transparent oval cells are present (Fig. 22).
Esparto.—Fibre 1.5 mm. x .012 mm. Stained greyish-blue to colourless. The fibres are very fine and short with pointed ends. Characteristics of esparto are the comma-shaped hairs and the serrated cells (Fig. 23).
Bamboo.—Fibre 4 mm. x .015 mm. Stained yellow to pale brownish-green. Resembling esparto, with cylindrical fibres with pointed ends, and usually a large number of transparent oval cells are found in paper made from bamboo (Fig. 24).
Chemical Wood.—The fibres vary considerably in length and thickness. Stained blue to colourless. Consisting of flat ribbon-like fibres, broad flat cells pitted and perforated, others similar to sections of a plant stalk, they are on the whole unlike any other fibres. A few fibres resemble linen fibres, but comparison will reveal differences. The differentiation between pine, spruce, poplar, birch is unnecessary for ordinary paper testing (Fig. 25).
Mechanical Wood.—Stained yellow. This pulp is unmistakable, owing to the broken pieces of various sizes and shapes, fragments of fibres torn away from the original wood, held together by cells, and showing pits and pores. Most newspapers are made of a mixture of chemical and mechanical wood, and microscopic examination of these mixtures furnishes an easy way of becoming familiar with the appearance of the different wood pulps (Fig. 26).
To arrive at a correct result, as regards the proportion of fibres in a mixture, is not at all easy. By taking a series of fields on one slide, counting and tabulating the contents under the headings of the different fibres, and averaging the fields, a fair approximation can be obtained. For comparison of two or more papers this will usually suffice, but consider-
