In 1889 Mr. Allan Dick brought forward the instrument already referred to (fig. 15), the details of which were worked

and cross-wires

out by Messrs. Swift & Son, of London. The stage is fixed and A new form of microscope." Mineralogical Magazine, vol. viii., p. 160. A complete description by Mr. Dick is also published by Messra. Swift & Son.

of an ordinary compact size; while the eye-piece with its cross-wires, and bearing the analyser above it, is connected by toothed wheels and a rod with the polariser below the stage. With the finger touching one of these toothed wheels, the observer causes the nicols and cross-wires to rotate together, and the vibration-directions of the nicols are thus brought parallel to particular directions in the crystal under examination, an arrangement that renders unnecessary the rotation of the crystal itself. The necessity for adjusting any particular power, however high, until it is exactly centred is thus entirely done away with.

The converging lens slides in and out in a groove cut in the stage; and a number of other ingenious details make this petrological microscope the most remarkable that has appeared since that of Nachet, and certainly the compactest and most convenient of any at present in the field. It is adapted, however, to higher requirements than those of most geological students, for whom the various types of strong instruments with well centred stages provide all that is necessary in ordinary work.

It is often useful to examine small crystals or grains by reflected light under a power of 1-inch or 1-inch focus. For this purpose the little support suggested by Mr. T. H. Holland* serves admirably and is easily made. A hemisphere is cut out of cork, about inch in diameter, and a small hollow is sunk in the centre of the flat side. The object to be examined is fixed in this hollow by a little wax so that it occupies the exact centre. If the hemispherical surface is allowed to rest in the circular aperture of the stage of the microscope, the support may be turned about in all directions without the object passing out of view or out of focus. Mr. Holland suggests a further use for this support. By turning out a conical hole completely through the hemisphere, widening below, "this simple contrivance may be employed in transmitted light, for adjusting sections of crystals which are supposed to have been cut in any particular direction; but which, as is almost always the case, only approximate what they are represented to be. Suppose, for example, a section of a doubly refracting uniaxial crystal is required, normal to the optic axis. Then by employing the hemisphere, the section can be so adjusted that its optic axis is exactly parallel to the line of collimation of the instrument."

Small objects such as sand-grains may be examined dry upon a glass slip, or by transmitted light when mounted in water under a cover-glass. Should it be desirable to preserve the * Science Gossip, No. 291 (March, 1889), p. 53.

specimens, objects for reflected light should be mounted as follows:-A slip of cedar-wood or mahogany, of the same size as the ordinary microscopic slide, and slightly thicker than the objects to be examined, has a circular hole bored through its centre. It is then backed by a thin piece of card blackened on the side that shows through the aperture. The objects may be fixed by minute drops of gum to this dull black ground in any suitable position. Care must be taken to prevent the gum from rising on to the sides of the objects and imparting to them an artificial appearance of polish. Tiny supports of blotting-paper soaked in gum can be folded into any shape when moist, and will prop up a minute specimen in any required position.

For use with transmitted light, the objects are mounted in Canada balsam, which has a refractive index sufficiently high to prevent the dark-bordered effect and imperfect transparency noticeable when air or even water surrounds the specimens. With delicate objects the balsam must be diluted with benzole and the cover-glass gently laid on the top. The slide must then be dried at a temperature of about 72° C., the spirit being thus driven off. To ascertain if the process has gone sufficiently far, pick up from time to time with a splinter or match-stick a little of the balsam exuded round the edges of the cover-glass. Directly the sample thus removed is cold, press it with the thumb-nail. If it is brittle, the whole balsam will become hard and firm on cooling, and the slide may be removed from the airbath in which it has been drying.

If the objects are strong, like sand-grains, the balsam may be dried previous to use. A little is softened by heat, and the objects are allowed to sink in it on the slide. The cover-glass is laid on while all is warm, pressed down, and the slide is laid aside to cool. Care must be exercised to prevent bubbles from forming in the balsam or the mass from becoming too brittle and discoloured by over-heating.

Very often the pressure used in placing the cover-glass in position forces the loose objects outwards or to one side of the preparation. Mr. H. C. Sorby* recommends the following remedy:-" Having placed a small quantity of dissolved gum on the glass plate, the requisite amount of the deposit is taken and mixed with the gum and sufficient water to make it easy to separate the grains and spread them uniformly over the space which will afterwards be covered by the thin glass. The water is then allowed to evaporate slowly, and though much of the gum collects round the margin, by properly regulating the quantity * Presidential address to Royal Microscopical Society, 1877.

originally added, enough remains under the larger grains to hold them so fast that they are not squeezed out with the excess of balsam."

The gum should be in so small an amount as to be invisible, after the mounting is completed, except on an examination of the under side of the preparation.

It is scarcely necessary to remind workers that the water used in any treatment of microscopic objects must be distilled, since on the evaporation of ordinary water all manner of crystalline salts are deposited upon the objects.

To remove the balsam exuded round the edges of the coverglass, the whole is allowed to become hard; the more prominent parts may then be chipped off readily with a blunt knife, the point of which must be kept clear from the edge of the cover; while the slide is finally and neatly cleaned with a tooth-brush dipped in methylated spirit. Wipe and dry at once with a soft duster.

Where microscopic sections are required, the preparation of the object is often a matter of considerable time. In the case of fragmental substances, an ingenious method suggested by Dr. Wallich may be applied after a little practice. A small glass slip, or square cut from plate glass, is laid upon a metal plate over a spirit-lamp. A drop of nearly dried balsam is softened upon this by heat, and a plate of mica is laid on it, becoming thus cemented to the glass. The small objects of which sections are to be prepared are then embedded in further balsam upon the mica surface and arranged in any suitable position. When this balsam is cold and firm, the glass is used as a handle by which the objects can be held during grinding. A flat surface is then given to them as they lie in the balsam by rubbing with water on a hone made of Water-of-Ayr stone. When they appear properly rubbed down, the surface is washed and dried, and the glass is again lightly heated. As soon as the heat softens the lower film of balsam, that between the mica and the glass, the mica must be lifted up with a pair of forceps and turned over on to an ordinary microscopic slip on which a little balsam has been heating. The ground surface of the objects is now downwards, and they become fixed on cooling to the glass slip in the positions originally selected. When cold, use the new slip as a handle, flake off the mica lightly with a knife, since it now forms the surface-layer, and grind the newly exposed side of the objects in the same manner as before. When transparent sections have been thus prepared, wash, soften the balsam by gentle warming, and affix the cover-glass. For details concerning the making of

* Ann. and Mag. Nat. Hist., 3rd ser., vol. viii. (1861), p. 58.

sections of delicate objects by such methods, see Chapman (Scientific News, vol. i., London, 1888, p. 452) and Stöber (Travaux du laboratoire minéralogique de l'université de Gand, 1899).

The same principles are involved in the preparation of sections of minerals and rocks such as are ordinarily examined by the geologist. A conveniently thin chip is chosen, and a smooth surface is given to it on one side. For this purpose it is held in the hand and rubbed down with emery and water on a plateglass, lead, or iron surface. Emery, of the coarseness known as "60-hole," may be used at first; then "90-hole," and finally "flour-emery," or even emery of greater fineness produced by washing. The emery already used must be carefully washed off from the specimen before proceeding to a finer quality. When objects, such as the bars of a window, can be seen reflected in the prepared surface, this first stage of the process may be considered as finished. The chip is now cemented, the polished surface downwards, by means of Canada balsam to a glass handle, preferably a piece of plate-glass about 14 inches square. When the cement has become hard, no trace of bubbles should appear between the surfaces of the specimen and the glass. If present, they will gleam, as seen through the glass, like little mercurial globules, and must be got rid of by further heating and probably by the application of more balsam. The surface thus finished and cemented down forms one side of the section ultimately prepared.

The outer side of the specimen is now similarly ground away, flour-emery alone being used when the section begins to appear translucent, and the finishing touches being given with the Water-of-Ayr stone. Care must be taken in order that the central area, and not merely the edges, may become finally thin. Very delicate sections must be transferred at an early stage to the slip on which they are to be mounted, and finished on this instead of on the handle of plate-glass.

In ordinary cases, however, the section is examined under the microscope before removal from the glass handle, to see if it is sufficiently and uniformly thin. It is then placed in a bath of methylated spirit for some five or six hours, at the end of which time it can be floated off from the handle and transferred carefully by a section-lifter (a bent piece of thin metal in a wooden handle) to an ordinary slip on which balsam is already heating. The covering and final processes are as described on pp. 128, 129.

* Prof. Rosenbusch recommends 16 parts by weight of Canada balsam to 50 parts of shellac, heated together for some time. The toughness of this allows very thin sections to be made.