As we proceed, the student will see more and more the practical bearings of this branch of the study, and will perceive the inferences to be drawn from the investigation of {35} minute relationships to manufacturing processes and their products. [Illustration: FIG. 4.] We shall say but little as to the necessary equipment. (1) A dissecting microscope, for dissecting under a lens, magnifying the object to 40 or 50 diameters. (2) An ordinary student’s microscope with lenses for magnifying to 100 and 300 diameters. This is adequate to the work, though of course, it may be an advantage in certain cases, to be provided with higher powers. (3) A glass slide, carrying an engraved scale of centimetres and millimetres for measuring the lengths of objects, and a micrometer, divided into 1⁠/⁠10 mm. for measuring diameters. It is also important to be able to determine the degree of enlargement under any particular combination of lenses, and for this purpose to possess a micrometer eye-piece, with a millimetre scale divided into hundredths. (4) An effective microtome and the usual mounting accessories. A very important feature in the diagnosis of fibres, more especially in regard to the composition of the fibre substances, is the effect produced by treatment with various reagents. Certain of these reactions we have already {36} indicated. We shall now give the details of composition of the several solutions which will be required. _Neutral Mounting Solutions._—It is advantageous in mounting transparent objects to employ a solution of the same refractive power as the substance itself. For this purpose, pure glycerin or a syrupy solution of calcium chloride may be employed; it is expedient to mix with either reagent a small proportion of acetic acid. The designation “neutral” has reference to the fact that these reagents are without sensible action on the fibres. _Iodine Solution._—We have previously described the preparation of a solution, giving the characteristic blue reaction with cellulose directly. It is, however, often preferable to bring about this colouration in another way, and the following are the solutions employed:—1 gramme of potassium iodide is dissolved in 100 cc. water, and the solution is saturated with iodine; it is preserved in stoppered bottles, containing a few fragments of the element, so as to keep up the saturation of the solution. The accessory solution, dilute sulphuric acid, which is employed to determine the reaction between the cellulose and the iodine, is prepared as follows:—2 volumes of concentrated glycerin are mixed with 1 volume water, and to the mixture an equal volume of oil of vitriol (1·78 sp. gr.), is slowly added, so as to prevent as far as possible a rise of temperature. The effect of the glycerin is very remarkable in preventing the distortion of the objects under the action of the acid, which in other respects remains uninfluenced.