merely a square plate with a large hole in the centre, through which the light can be reflected from a mirror N into the microscope. The mechanical stage, as shown in the illustration, is provided with two adjustments, by means of which the body of the stage can be moved horizontally from side to side or from back to front. This movement is most useful in the examination of fibres, though not essential, as it enables the operator to bring systematically into the field of observation all the fibres which may be mounted on the slide. Underneath the slide is the reflecting mirror N, which is flat on one side and concave on the other. This mirror is used for reflecting the light from the window through the object on the stage into the tube of the instrument. The mirror is fitted on a swivel P, so that when the instrument is in the horizontal position it can be turned aside to permit of the direct light from a lamp being passed through the slide. The sub-stage S is used for carrying the various additional appliances, such as the sub-stage condenser, which is necessary in some cases for the examination of objects under high powers. For special work in the examination of fibres the sub-stage is used, for example, when the fibres are being examined under polarised light. Accessories. There are a number of additional appliances which should be obtained by those intending to carry out any prolonged study of fibres. Nose-piece. This is a simple contrivance that can be attached to the lower end of the body-tube, by means of which various objectives can be quickly Fig. 129.-A Double Nosepiece for carrying two Objectives. exchanged. It is constructed to carry two or three objectives, each of which can be brought into action by a simple rotation of the nose-piece. A double nose-piece is quite sufficient for most purposes. Millimetre scale.-This consists of a glass slide of the usual dimensions, three inches by one, on which is etched a line divided off by cross lines into equal divisions of one-tenth of a millimetre. One of the spaces is further sub-divided into tenths. Eye-piece micrometer. This in its simplest form is a small circle of glass, which fits into any one of the eye-pieces. A number of arbitrary divisions marked 0 to 10 are etched upon the glass. The micrometer is used in conjunction with the millimetre scale for the measurement of the length and diameter of fibres. Beale's tint reflector. This is a useful contrivance, by means of which a drawing can be made of the object Fig. 131.-Beale's Tint are plenty of special lamps Fig. 130. An Eye- The microscope is examined. It consists of a Lamp. This is only required for work done when good daylight is not available. For ordinary purposes a common 18. oil lamp will do, though there sold for microscope work. Slides. Three or four dozen glass slides of the usual type, measuring three inches long by one inch wide. Cover-glasses. These are supplied in boxes and are either square or circular in shape, the most suitable size being five-eighths or three-quarter inch. Microscope needles.-A few microscope needles, straight or bent, the latter being more convenient. If the Drainer.-A small piece of fine brass wire gauze for draining off water from the fibres. An old piece of paper machine wire about three inches square, with the edges doubled over and flattened down answers the purpose admirably. piece of wire is placed over the mouth of a small bottle and Fig. 132.--Drainpressed down with the thumb, a slight ciring Sieve. cular depression can be made in the wire cloth, which thus forms a convenient receptacle for the disintegrated pulp. Fig. 133.Dropping Bottle with Grooved Stopper. Reagent bottles.-Special bottles with groove stoppers so constructed that the liquid is discharged from the bottle in single drops. Cabinet.-A small box capable of holding twenty or thirty slides, which can be kept for reference. Porcelain basins. -Two or three flat-bottomed shallow dishes about three to four inches diameter will be sufficient. Spirit lamp. This will be useful for boiling papers and fibres in caustic soda solution. Fig. 134. Spirit Lamp. Place a ready prepared slide, preferably one containing some permanently mounted fibres, which can be purchased at any optician's or microscope dealer's, on the stage. Fit the half-inch objective into the lower end of the body-tube, and one of the eye-pieces into the upper end of the tube. Proceed to focus the objective by means of the coarse adjustment, an operation which will require some care. Turn the milled heads of the coarse adjustment so as to bring the objective upwards away from the object until the distance of the objective from the slide is about one inch. Then placing the eye to the instrument slowly rack down the tube until the blurred image comes into view, and continue the movements very cautiously until the image is clearly focused. When the slide contains only a few fibres distributed all over the field of view, as it is called, there is sometimes a danger that if none of the fibres happen to be in the direct line of vision the observer may rack down the tube hard on to the microscope slide and injure the objective, or the object. This danger must be guarded against by careful manipulation. The following course might be adopted by a beginner. Place the slide in position. and rack down the tube until the objective is close to the slide, watching the distance between them very carefully to avoid actual contact. Then, placing the eye to the instrument, turn the milled heads of the coarse adjustment so that the focusing is produced by the upward motion of the body instead of the downward motion as previously described. Effect of Varying the Powers. Select some particular fibre of definite structure amongst those upon the slide. Fix the slide in position on the stage by means of the stage springs and proceed to note the differences in appearance produced by the various eye-pieces and objectives available. For example, observe the appearance of a fibre with the lowest magnifications, and note the increase of size due to elongation of the tube to its full length. Examine the same fibre with higher objectives, observing not only the magnification but also the clearer definition of physical structure. Mounting Fibres. It is quite impossible in an elementary treatise of this description to enter fully into the whole subject of the mounting of fibres. structions must therefore be limited to those which will enable the student to examine fibres and papers for technical purposes. Taking the case of an ordinary high-class rag paper the student should proceed as follows: From the sheet of paper cut three or four pieces about a half-inch square, so as to obtain a representative sample. Place the paper in a small porcelain dish with sufficient caustic soda solution (1 per cent. solution will be strong enough) completely to cover the paper. Gently warm the dish with its contents in any convenient manner for about half an hour, in order to dissolve out the gelatine and other sizing ingredients. Remove the pieces of paper from the dish and thoroughly wash them free from caustic soda. Then put the paper in a small bottle together with some glass beads, and nearly fill the bottle with water. Shake the bottle vigorously until the whole of the paper is completely disintegrated into isolated fibres and then strain off the contents of the bottle through a piece of very fine wire gauze or muslin. The water drains away leaving the fibres on the gauze in a condition fit for examination. Fig. 135.- With two suitable microscope needles, pick out a few fibres here and there and transfer them to an ordinary glass slip, removing the excess of moisture by pressing the little heap of fibres lightly with a piece of filter paper. Moisten the sample with one or two drops of some microscope reagent, preferably the zine chloride and iodine solution. Distribute the fibres evenly on the glass by tapping them gently with a glass rod, the end of which has been carefully rounded off. The distribution of the fibres is a very important matter. The beginner invariably places too large a quantity of pulp on the slide, and renders it almost impossible to make a thorough and adequate examination of the fibres. A small quantity of pulp evenly distributed on the glass, avoiding the aggregation of small masses, gives far better results, as experience will soon prove. Cover the stained fibres with a circular cover-glass, five-eighths or threequarters of an inch in diameter, taking care that no air bubbles are imprisoned under the glass. This is most easily effected by holding the cover-glass with a needle in a slanted position on the slide close to the fibres, and lowering the glass gradually down upon the mass of fibres, as in this way the liquid comes into contact with the glass and drives out all the air as the glass is lowered. Remove any excess of reagent or liquid by what is known as the process of irrigation—that is to say, by holding the edge of a piece of filter paper on the slide close up to the cover-glass. The excess liquid is soaked up by the filter paper. Finally, press down the cover-glass gently with the filter paper, so as to bring all the fibres flat down upon the glass. The fibres thus mounted may be transferred to the microscope and examined, Examination of New Fibres.-A small quantity of pulp for microscopic examination can readily be obtained from any raw material by the following . simple process: Break up a piece of the material, if necessary, into small pieces. The best method of doing this is to hammer the fibre gently on a block of wood, or a piece of stout iron plate, until it is frayed out into a more or less fibrous condition. Place the disintegrated fibre in a flask or a small saucepan, and cover it with a solution of caustic soda (10 parts of caustic soda in 100 parts water), and boil for about an hour, taking care to make up the loss due to evaporation of the water by adding small quantities of fresh water from time to time. When the fibre is soft, pour off the water through a wire cloth and thoroughly wash the small quantity of fibre. The pulp so obtained can be used for microscopic investigations. If bleached pulp is desired, put the boiled fibre in a solution of chloride of lime, and leave the mixture to stand all night. Drain off the water as before, and thoroughly wash with clean water. ence. Estimating Percentages of Fibre.-The examination of a paper containing two or more fibrous constituents is a matter which demands considerable experiIt is scarcely possible to determine the proportions present with a degree of accuracy greater than 5 per cent. in normal cases. A sample of paper is broken up and mounted in the usual way, and the proportions of fibres arrived at by one of two methods: (1) The method of "counting the fibres." (2) The method of "mental impressions." In the first case various portions of the slide are brought into the field of view and the fibres counted, due allowance being made for irregularity in length. In the second case the fibres are not absolutely counted, but a record is made for each field of view examined as to the impression made on the mind of the observer. The beginner is recommended to make estimates as to the percentages of two fibres present, by using mixtures of known composition, for there is no royal road to success in this work except that of patience and perseverance with mixtures in which the proportions are already known. Some Simple Exercises. (A) Select an antique paper known to consist of esparto and chemical wood pulp, and disintegrate the paper for examination. As a staining reagent, use zinc chloride and iodine solution. Notice effect produced on the depth of colour due to the reagent when using the latter in various stages of dilution as follows: Mount some of the moist pulp with one drop of reagent. Mount some of the pulp, removing the excess of water from the fibres before staining by the process of irrigation, that is, by bringing a piece of filter paper into contact with the pulp and allowing the moisture to be absorbed as far as possible; stain with one drop of zinc chloride and iodine solution. Mount the third slide, using an equal quantity of pulp, removing all the moisture as far as possible by pressing down a piece of filter paper on the fibres. Then add one drop of zinc chloride and iodine reagent and mount as before. (B) Changing the reagent and using iodine solution proceed as follows: Place equal quantities of fibre on four slides and allow the pulp to dry out in the air. Add one drop of iodine solution to the first slide and cover as usual. To the second add one drop of iodine solution diluted with one drop of water. To the third add one drop of iodine solution diluted with two drops of water. Mount the remaining slides, diluting the iodine solution with water in larger proportions, and finally compare the difference in the intensity of colour. (C) Make notes as to the physical structure of the esparto and wood pulp fibres, comparing the notes so made with the description given in chap. xii. Search for all the characteristics of the fibres, and as far as possible make drawings of those examined. (D) Estimate the proportions of fibres present, going over the whole slide carefully, taking about twenty or thirty readings, in each case adopting the method of "counting the fibres," and the method of "mental impressions." The Measurement of the Length of Fibres.-The length of fibres is determined by means of the stage micrometer and the eye-piece micrometer, which instruments have already been described. The millimetre scale is placed on the stage of the microscope in the usual way, and the eye-piece scale placed in the eye-piece. The microscope is then focused clearly, and the divisions of the millimetre scale and eye-piece scale can then be observed together as if in one field. The eye-piece is rotated until its divisions run parallel with the divisions on the millimetre scale, and when this has been done a careful note is made as to the number of millimetres corresponding to the ten divisions of the eye-piece scale. For example: Divisions on the eye-piece scale, 10·0. Divisions on the millimetre scale, 0·69. From this it follows that each whole division on the eye-piece scale is equivalent to 069 millimetre. This value is, of course, only true for the conditions under which the readings have been taken, and for the particular combination of eye-piece, objective and tube length concerned. Any alteration in one or more of these affects the values of the eye-piece divisions. A stage micrometer divided into thousandths of an inch can be used if preferred. Thus, for example, with one inch objective and a tube length of 10 inches for a particular eye-piece, the value may be 7 divisions, on eye-piece scale = 483 millimetre. If the tube length is reduced to 6 inches by being pushed in, then 7 divisions on scale may equal 770 millimetre. It is therefore necessary to compile a table, stating clearly the conditions under which the microscope is used, and the value of the scale for all the conditions possible. TABLE XXVI. Of Values for Microscope (Messrs. Watson. Stand H.) Showing the Value of One Whole Division of Eye-piece. Scale in Millimetres. |