Common Objects of the Microscope. John George Wood

is almost the most valuable. So much may be effected by its means alone, in practised hands, that it is well worth the while of the beginner to master its use thoroughly, and the methods of doing so will be explained in the succeeding chapter.

      The substage condenser, too, even in its most simple form, is an invaluable adjunct, even though it be only a hemisphere of glass, half an inch or so in diameter, mounted in a rough sliding jacket to fit underneath the stage. Such an instrument, properly fitted, will cost about fifteen shillings, but the ingenious worker will easily extemporise one for himself.

      Fig. 11.

      Many plants and animals require to be dissected to a certain extent before the details of their structure can be made out, and for this purpose the naked eye alone will rarely serve. The ordinary pocket magnifier, however, if mounted as described in a preceding chapter, will greatly facilitate matters, and the light may be focused upon the object by means of the bull’s-eye condenser, as shown in Fig. 11. In the figure the latter is represented as used in conjunction with the lamp, but daylight is preferable if it be available, the strain upon the eyes being very much less than with artificial light. Two blocks of wood, about four inches high, will form convenient rests for the hands, a plate of glass being placed upon the blocks to support the dish, and a mirror being put in the interspace at an angle of 45° or so if required. A piece of black paper may be laid upon the mirror if reflected light alone is to be used.

      As all delicate structures are dissected under fluid, a shallow dish is required. For this purpose nothing is better than one of the dishes used for developing photographic negatives. The bottom of the dish is occupied by a flat cork, to which a piece of flat lead is attached below, and the object having been pinned on to the cork in the required position, the fluid is carefully run in. This fluid will naturally vary according to the results desired to be obtained, but it must not be plain water, which so alters all cellular structures as to practically make them unrecognisable under the microscope. Nothing could be better than a 5 per cent. solution of formalin, were it not for the somewhat irritating odour of this fluid, since it at once fixes the cells and preserves the figure. For many purposes a solution of salt, in the proportion of a saltspoonful of the latter to a pint of water, will answer well for short dissections. For more prolonged ones, a mixture of spirit-and-water, one part of the former to two of the latter, answers well, especially for vegetable structures. When the dilution is first made, the fluid becomes milky, unless pure spirit be used, but with a little trouble the Revenue authorities may be induced to give permission for the use of pure methylated spirit, which answers every purpose. The trouble then is that not less than five gallons can be purchased, an embarras de richesses for the average microscopist, but, after all, the spirit is extremely cheap, and does not deteriorate by keeping.

      When the dissection in either of these media is completed, spirit should be gradually added to bring the strength up to 50 per cent., in which the preparation may remain for a day or two, after which it is gradually brought into pure spirit, or into water again, according to the medium in which it is to be mounted.

      Fig. 12.

      As to the tools required, they are neither numerous nor expensive. Fine-pointed but strong forceps (Fig. 9, c), curved and straight; a couple of pairs of scissors, one strong and straight, the other more delicate, and having curved blades, and a few needles of various thicknesses and curves, are the chief ones. The latter may be made by inserting ordinary needles, for three-fourths of their length, into sticks of straight-grained deal (ordinary firewood answers best), and thereafter bending them as required. A better plan, however, is to be provided with a few of the needle-holders shown in Fig. 9, b. These are very simple and inexpensive, and in them broken needles are readily replaced by others. Dipping-tubes, such as are shown in Fig. 12, will also be extremely useful for many purposes. These are very easily made by heating the centre of a piece of soft glass tubing of the required size, and, when it is quite red-hot, drawing the ends apart. The fine tube in the centre should now be divided by scratching it with a fine triangular file, and the scratch may of course be made at such a point as to afford a tube of the required fineness. The edges should be smoothed by holding them in the flame until they just run (not melt, or the tube will close). These tubes can often be made to supply the place of a glass syringe. They may be used either for sucking up fluid or for transferring it, placing the finger over the wide end, allowing the tube to fill by displacement of air, and then re-closing it with the finger. This last method is especially useful for transferring small objects from one receptacle to another. In speaking of the dissection of objects, it should have been mentioned that the microscope itself may, under careful handling, be made to serve very well, only, as the image is reversed, it is almost impossible to work without using a prism to re-erect the image. Such a prism is shown in Fig. 13. The microscope is placed vertically, and the observer, looking straight into the prism, sees all the parts of the image in their natural positions. This appliance is extremely useful for the purpose of selecting small objects, and arranging them on slides in any desired manner. A few words may be added as to the reproduction of the images of objects.

      Fig. 13.

      The beginner is strongly recommended to practise himself in this from the outset. Even a rough sketch is worth pages of description, especially if the magnification used be appended; and even though the worker may be devoid of artistic talent, he will find that with practice he will acquire a very considerable amount of facility in giving truthful outlines at least of the objects which he views. Various aids have been devised for the purpose of assisting in the process. The simplest and cheapest of these consists of a cork cut so as to fit round the eye-piece. Into the cork are stuck two pins, at an angle of 45° to the plane of the cork, and, the microscope being placed horizontally, a thin cover-glass is placed upon the two pins, the light being arranged and the object focused after the microscope is inclined. On looking vertically down upon the cover-glass, a bright spot of light will be seen, and as the eye is brought down into close proximity with it the spot will expand and allow the observer to see the whole of the image without looking into the microscope. If a sheet of paper be now placed upon the table at the place occupied by the image so projected, the whole of the details will be clearly seen, as will also the point of a pencil placed upon the paper in the centre of the field of view; and, after a little practice, it will be found easy to trace round the chief details of the object. Two points require attention. The first is that if the light upon the paper be stronger than that in the apparent field of the microscope, the image will not be well seen, or if the paper be too feebly lighted, it will be difficult to keep the point of the pencil in view. The light from the microscope is thrown into the eye, and the view of the image upon the paper is the effect of a mental act, the eye looking out in the direction from which the rays appear to come. The paper has therefore to be illuminated independently, and half the battle lies in the adjustment of the relative brightness of image and paper. The second point is, that it is essential to fix one particular point in the image as the starting-point of the drawing, and this being first depicted, the image and drawing of this point must be kept always coincident, or the drawing will be distorted, since the smallest movement of the eye alters the relations of the whole. The reflector must be placed at an angle of 45°, or the field will be oval instead of circular. The simple form of apparatus just described has one drawback, inasmuch as the reflection is double, the front and back of the cover-glass both acting as reflectors. The image from the latter being much the more feeble of the two, care in illumination will do much to eliminate this difficulty; but there are various other forms in which the defect in question is got rid of. The present writer has worked with all of them, from the simple neutral tint reflector of Beale to the elaborate and costly apparatus of Zeiss, and, upon the whole, thinks that he prefers the cover-glass to them all.

      A very simple plan, not so mechanical as the last-named, consists in the use of “drawing-squares,” which are delicate lines