SECTIONS AND HOW TO CUT THEM, WITH SOME REMARKS ON DISSECTION.  
Many objects are almost worthless to the microscopist until the extraneous matter is removed from them; and this is frequently difficult in the extreme to perform satisfactorily. As an instance, certain Foraminifera may be mentioned in which the cells are placed one upon another, consequently the object must be reduced to a certain degree of thinness before a single uniform layer of these cells can be obtained to show something of the internal arrangements.
Most animal and vegetable forms require an examination of the separate parts before much can be known about them. The mass must be divided into separate portions, each part intended to be preserved being cleaned from the useless matter with which it is surrounded. It will frequently be found necessary to make thin sections, which from a very tender substance is no easy matter; and much patience will be necessary to attain anything like proficiency.
This making of sections was not until very recently entered into by many except those belonging to the medical profession, but I do not see why this should be so, as much may be accomplished by a persevering and interested mind where there is time for entering into the subject. I will therefore make an attempt to give some instructions on this subject also. We will first consider the cutting of sections from hard substances, in which the ordinary knife, chisel, &c., are of no avail. Most of these require no particular care in mounting, but are placed in balsam like the other objects noticed in Chapter III.: where, however, any special97 treatment is necessary it will be commented upon as we proceed.
Shells, &c.—It is seldom, if ever, necessary to possess apparatus for this process except a small thin saw made with a steel blade, for which a piece of watch-spring serves very well; a fine stone such as is used for sharpening pen-knives; and two smooth leather strops, one of which is to be used with putty-powder to polish the section after grinding, and the other dry, to give the final surface. It is, however, very convenient to have three or four files of different degrees of fineness. The shell, if very thick, may be divided by using the watch-spring saw; and this section may then with ordinary care be rubbed down with water on the stone until one side of it is perfectly flat. When this is accomplished it must be again rubbed with putty-powder upon the strop, and finally upon the other strop without the powder. This surface will then be finished and must be firmly united to the slide in the position it is intended to occupy. To do this a small quantity of Canada balsam may be dropped upon the middle of the slide and heated over the lamp until on cooling it becomes hard; but this must be stopped before it is rendered brittle. Upon this the polished surface must be laid, and sufficient heat applied to allow the object to fall closely upon the slide, when slight pressure may be used to force aside all bubbles, &c. On cooling, the adherence will be complete enough to allow the same grinding and polishing upon the upper surface which the lower received. Whilst undergoing this, the section must be examined from time to time to ascertain whether the necessary degree of thinness has been reached. When this is the case the section should be washed thoroughly and dried. It must then be covered, which is best done by using the ordinary Canada balsam, as recommended in Chapter III.
Sections of some exquisitely beautiful objects are cut with much less trouble than the above. The Orbitolite, for instance, may be prepared in this manner. Take the object and by pressure with the finger rub the side upon98 a flat and smooth sharpening stone with water until the portion is reached which it is wished to show. The strength of the object will easily allow this to be accomplished with ordinary care. This side may then be attached to the glass slide with heated balsam, as above described, and the object may then be gently rubbed down to the degree of thinness required to show it to the best advantage. After removing all disengaged matter from the object by washing and thoroughly drying, it may be mounted in balsam in the usual manner, when it is equally beautiful as a transparent or opaque object. From this it will be seen that in many instances where a smooth stone is found sufficient for the work (which is often the case when the section is mounted in balsam) the final process of polishing advised above may be dispensed with, as in the Orbitolite, Nummulite, &c., &c. It is quite necessary that the stones on which the objects are rubbed be perfectly flat, otherwise one side must be acted upon before the other, and it will be found impossible to attain anything like uniformity. Where it is not practicable to cut a section and the object is very thick, a coarse stone may be first used to reduce it and the smoother afterwards.
The consideration of the cutting of sections from shells would scarcely be deemed complete without some mention of what Dr. Carpenter terms the decalcifying process. Muriatic acid is diluted with twenty times its volume of water, and in this the shell is immersed. After a period, differing according to the thickness of the shell, the carbonate of lime will be dissolved away, and a peculiar membrane left, showing the structure of the shell very perfectly. This may be mounted dry, in balsam, or sometimes in liquid, according to the appearance of the object; but no rule can be given. The discretion of the student, however, will enable him to choose the most suitable method.
From some shells it is easy to divide thin plates, or “lamin?,” which require nothing but mounting in Canada balsam to show the texture very well. In working, however,99 with those which are “pearly,” it will be found that experience and patience are needed, as they are very brittle and peculiarly hard; but a little practice will overcome these difficulties.
Amongst the Echinodermata, which include the star-fishes, sea-hedgehogs, &c., there are many whose outer surface is covered with “spines,” or thin projections. Some of these are sharp and thorn-like, others blunt, longer or shorter, and, indeed, of endless variety. In many of these, when a section is made, rings are seen which have a common centre, with radiating supports, resembling sections of some of the woods. These are very beautiful objects, and methods of procuring them may now be considered. It is the best to cut as thin a section as can safely be got with the watch-spring saw first, when the smooth “sharpening stone” may be used to polish one side, which is easily accomplished with water only. When this is effected, it must be washed clean, and thoroughly dried, and then may be united to the slide in the same manner as before recommended for the Orbitolite, &c. If it is ever necessary to displace it on account of inequalities, bubbles, or other remediable fault, this may be done by warming the slide; though too much heat must be avoided, otherwise fresh bubbles will certainly be produced. The covering with thin glass, balsam, &c., will present no difficulty to the student; but he must remember that the transparency is somewhat increased by this last operation.
Corals are often treated in this way, in order to reveal their structure. Except, however, the student has had much practice, he will often find this a most difficult task, as many of them are exceedingly brittle. He will find the method before described equally applicable here, and should take both horizontal and vertical sections.
Coal.—This substance is one of the most interesting objects to the microscopist. It is, of course, of a vegetable origin; and though this is in many cases of such minute separate portions as to have lost all appearance of vegetation,100 yet it is very frequently met with in masses, bearing the form, even to the minute markings, of wood, in various directions. To see this and prepare it for microscopic research, a suitable piece of coal must be obtained; but in every case the cutting and preparation of these sections require great care and skill. Sometimes the coal is first made smooth on one side, fastened to the glass, reduced to the requisite degree of thinness, and finished in the method before described. This mode of treating it is sometimes, however, very tantalizing, as, at the last moment, when the section is about thin enough, it often breaks up, and so renders the trouble bestowed upon it fruitless. The dark colour and opacity of coal render an extraordinary thinness necessary, and so increase the liability to this accident.
Perhaps the best method which can be pursued is that recommended in the “Micrographic Dictionary,” which is as follows:—“The coal is macerated for about a week in a solution of carbonate of potash; at the end of that time it is possible to cut tolerably thin slices with a razor. These slices are then placed in a watch-glass with strong nitric acid, covered and gently heated; they soon turn brownish, then yellow, when the process must be arrested by dropping the whole into a saucer of cold water, else the coal would be dissolved. The slices thus treated appear of a darkish amber colour, very transparent, and exhibit the structure, when existing, most clearly. We have obtained longitudinal and transverse sections of coniferous wood from various coals in this way. The specimens are best preserved in glycerine in cells; we find that spirit renders them opaque, and even Canada balsam has the same defect. Schulz states that he has brought out the cellulose reaction with iodine, in coal treated with nitric acid and chlorate of potash.”
Cannel-coal is so close and firm in its structure as to be much used instead of jet in the manufacture of ornaments: it takes a beautiful polish, and consequently presents the student with none but ordinary difficulties in getting101 sections of it. Its formation is somewhat different from that of coal, sometimes showing the transition very clearly.
In flint there are often found remains of sponges, shells, Diatomace?, &c.; but to show these well, sections must be cut and polished by the lathe and wheel of the lapidary, which the microscopic student seldom possesses. Thin chippings may, however, be made, which when steeped in turpentine and mounted in balsam, will frequently show these remains very well.
Teeth are very interesting objects to all microscopists, more especially to those who give much study to them; as the class of animal may very frequently be known from one solitary remaining tooth. To examine them thoroughly, it is necessary to cut sections of them; but this is rather difficult to perform well, and needs some experience. Some instructions, however, will at least lessen these difficulties, and we will now endeavour to give them.
It is generally thought that Canada balsam injures the finer markings of these sections, consequently, they are almost invariably mounted dry. A thin piece is first cut from the tooth with the saw of watch-spring before mentioned, if possible; but should the substance be too hard for this, the wheel and lathe must be used with diamond dust. If this cannot be procured, there is no alternative but to rub down the whole substance as thin as practicable on some coarse stone or file. The surface will then be rough; but this may be much reduced by rubbing upon a flat sharpening stone with the finger, or a small piece of gutta-percha, upon the object to keep it in contact. The scratches may be much lessened by this, but not so thoroughly removed as microscopic examination requires in dry sections. It must, therefore, be polished with the putty-powder and dry strop, as recommended in the working of the shell-sections. The other side of the section of the tooth may then be rubbed down to the requisite thinness, and polished in the same manner, when the dust and other impurities must be removed by washing, after which the section must be carefully dried102 and mounted. Some of these sections are equally interesting as opaque or transparent objects.
The dentine of the teeth may be decalcified by submersion of the section in dilute muriatic acid; after drying and mounting in Canada balsam it presents a new and interesting appearance, showing the enamel fibres very beautifully when magnified about three hundred diameters. A friend tells me that after submersion of the whole tooth in the acid he has been able to cut sections with a razor.
Sections of Bone.—With the aid of the microscope few fragmentary remains have proved so useful to the geologist and students of the fossil kingdom as these. From a single specimen many of our naturalists can tell with certainty to what class of animal it has once belonged. To arrive at this point of knowledge much study is necessary, and sections of various kinds should be cut in such a manner as will best exhibit the peculiarities of formation. The methods of accomplishing this will now be considered. It may, however, be first mentioned that the chippings of some bones will be found useful now and then, as before stated with flint, though this is by no means a satisfactory way of proceeding. Sometimes the bones may be procured naturally so thin that they may be examined without any cutting; and only require mounting dry, or in fluid, as may be found the best.
When commencing operations we must provide the same apparatus as is needed in cutting sections of teeth, before described. A fine saw, like those used for cutting brass, &c.; two or three flat files of different degrees of coarseness; two flat “sharpening” stones; and a leather strop with putty-powder for polishing. As thin a section as possible should first be cut from the part required by the aid of the fine saw; and it is better when in this state to soak it for some short time in camphine, ether, or some other spirit to free it from all grease. With the aid of a file we may now reduce it almost to the necessary degree of thinness, and proceed as before recommended with teeth. The103 “sharpening” stone will remove all scratches and marks sufficiently to allow it to be examined with the microscope to see if it is ground thin enough; and if it is to be mounted dry we must polish it with putty-powder and water upon the strop to as high a degree as possible, and having washed all remains of polishing powder, &c., from the section we must place it upon the slide and finish it as described in Chapter II.
If the bone is not sufficiently hard in its nature to bear the above method of handling whilst grinding and polishing—as some are far more brittle than others—as thin a section as possible must first be cut with the saw, and one surface ground and polished. The piece must then be dried and united to the glass by heated balsam in the same manner as shells, &c. After which the superabundance of balsam must be removed from the glass; then rub down upon the stone and strop as before. When the polishing is completed the whole slide must be immersed in chloroform, ether, or some other spirit, to release and cleanse the section, when it may be mounted as the one above mentioned.
Some have recommended a strong solution of isinglass to affix the half-ground teeth or bones to the glass as causing them to adhere very firmly and requiring no heat, and also being readily detached when finished.
The reason why the sections of bone are usually mounted dry is that the “lacun?,” bone cells, and canaliculi (resembling minute canals) show their forms, &c., very perfectly in this state, as they are hollow and contain air, whereas if they become filled with liquid or balsam—which does sometimes occur—they become almost indistinguishable. There are some dark specimens, however, where the cells are already filled with other matter, and it is well to mount these with balsam and so gain a greater degree of transparency.
To gain a true knowledge of the structure of bone, sections must be cut as in wood, both transversely and longitudinally; but with fossil bones, without the lapidary’s104 wheel, &c., it is a laborious task, and indeed can seldom be properly accomplished. In this place, also, it may be mentioned that by submitting bone to the action of muriatic acid diluted ten or fifteen times with water, the lime, &c., is dissolved away and the cartilage is left, which may be cut into sections: in caustic potash the animal matter is got rid of. Both of these preparations may be mounted in fluid.
The method of cutting thin sections of bone may be also employed with the stones of fruit, vegetable ivory and such like substances; many of which show a most interesting arrangement of cells, especially when the sections are transverse. Most of these objects present a different appearance when mounted dry to that which they bear when in balsam, owing to the cells becoming filled; and to arrive at a true knowledge of them we must have a specimen mounted in both ways.
To those who study polarized light, few objects are more beautiful than the sections of the different kinds of horn. We will briefly inquire into the best method of cutting these. There are three kinds of horn, the first of which is hard, as the stag’s, and must be cut in the same manner as bone. The second is somewhat softer, as the cow’s. The third is another and still softer formation, as the “horn” (as it is termed) of the rhinoceros. In cutting sections of the two last we should succeed best by using the machine invented for these purposes which I shall shortly describe when the method of cutting wood is considered. To aid us in this when the horn is hard it must be boiled for a short time in water, when the cutting will be more easily accomplished. The sections should be both transverse and longitudinal, those of the former often showing cells with beautiful crosses, the colours with the selenite plate being truly splendid. Of this class the rhinoceros horn is one of the best; but the buffalo also affords a very handsome object. The cow’s, and indeed almost every different kind of horn, well deserves the trouble of mounting. Whalebone, when cut transversely,105 strongly resembles those of the third and softer formation. All these are best seen when mounted in Canada balsam, but care must be taken that they have been thoroughly dried after cutting, and then steeped in turpentine.
An interesting object may also be procured from whalebone by cutting long sections of the hairs of which it is composed. Down the centre of each hair we shall find a line of cells divided from one another very distinctly. And (as recommended in the “Micrographic Dictionary”) if whalebone be macerated twenty-four hours in a solution of caustic potash it will be softened, and by afterwards digesting in water, the outer part will be resolved into numerous transparent cells, which will show more plainly the structure of this curious substance.
In a former chapter, hairs were mentioned, their many and interesting forms, and their beauty when used with polarized light. The sections of them, however, are no less a matter of study, as this mode of treatment opens to sight the outer “casing,” and the inner substance somewhat resembling the pith of plants.
It would be out of place to enter into the description of the different forms met with; but the ways in which sections are to be procured may be glanced at. If transverse sections are required, some place a quantity of hairs betwixt two flat pieces of cork, which by pressure hold them firmly enough together to allow the required portions to be cut with a razor. Others take a bundle of the hairs and dip it into gum or glue, which gives it when dry a solidity equal to wood. Sections of this are then cut with the machine mentioned a little further on, and these may be mounted in balsam. The human hair is easily procured in the desired sections by shaving as closely as possible a second time and cleansing from the lather, &c., by carefully washing. Most hairs, however, should be examined both transversely and longitudinally. It is not difficult to procure the latter, as we may generally split them with the aid of a sharp razor. In a great number of hairs there is a quantity of greasy matter106 which must be got rid of by soaking in ether or some other solvent before mounting.
We may next consider the best method of procuring sections of wood, which must be cut of such a degree of thinness as to form transparent objects, and so display all the secrets of their structure. There is no monotony in this study, as the forms are so various, and the arrangement of the cells and woody fibre so different, that the microscopist may find endless amusement or study in it. From a single section the class of trees to which it has belonged may be known, often even when the wood is fossil. The apparatus best adapted for cutting these sections is made as follows:—A flat piece of hard wood, about six inches long, four wide, and one thick, is chosen, to which another of the same size is firmly fixed, so as to form, in a side view, the letter T. On one end of the upper surface is fastened a brass plate, perfectly flat, in the centre of which a circular opening is cut about half an inch in diameter. Coinciding with this opening is a brass tube, fixed in the under side of the table (if it may be termed so). This tube is so cut at the bottom as to take a fine screw. Another screw is also placed at the same end of the “table,” which works at right angles to this, so that any substance in the tube may be wedged firmly by working this last screw. To use this instrument, the piece of wood or other object of which a section is required must be placed in the tube, when, by turning the screw underneath, the wood is raised above the brass plate more or less as wished, and by using the screw at the end, it is held firmly in the same position. With a flat chisel the portion of the object which projects above the surface of the brass plate may now be cut off, and by means of the bottom screw another portion may be raised and treated in the same manner. As to the thickness of which objects should be cut, no proper directions can be given, as this differs so greatly that nothing but experience can be any guide. The same thickness can be obtained by working the screw underneath in uniform107 degrees, the head being marked for this purpose; and when the substance to be cut is very much smaller than the hole in the brass plate, it may be wedged with cork.
As this instrument is peculiarly adapted for cutting wood (though used for other substances, as before mentioned), I shall notice a few particulars concerning this branch of sections. It may here be remarked, that to obtain anything like a true knowledge of the nature of wood, it should be cut and examined in at least two directions, across and along. The piece of wood is often placed in spirits for a day or two, so that all resinous matter may be dissolved out of it; it must then be soaked in water for the same length of time, so as to soften and render it easy to cut. Sections are obtained in the manner described above, but often curl to such a degree as to make it necessary to immerse them in water, from which they may be taken and dried under slight pressure. They are often mounted dry, and require no care beyond other objects, as in Chapter II. Some, however, are best mounted in balsam, particularly the long sections when used for the polariscope; these must be soaked in turpentine, and the greatest care taken that all air bubbles are got rid of. Others are thought to be most useful when mounted in shallow cells with some of the preservative liquids mentioned in Chapter IV.—weak spirit and water, chloride of calcium solution of the strength of one part of the salt to three parts of distilled water, &c.
The above “section-cutter” may not be within the reach of every student, nor is it absolutely necessary; though where any great number of specimens is required it is very useful, and insures greater uniformity in the thickness. Many employ a razor for the purpose, which must always be kept sharp by frequent stropping. Sections of leaves also may be procured by the same means, though, as before mentioned, they are sometimes easily divided by stripping the coatings off with the fingers. The cells which come to sight by cutting some of the orchideous108 plants are most interesting. To cut these leaves they may be laid upon a flat piece of cork, thus exposing the razor to no danger of injury by coming in contact with the support. It may be mentioned here that the razor may also be used in cutting sections of the rush, than which a more beautiful object can scarcely be found when viewed transversely, as it shows the stellate arrangements of the ducts to convey the liquids to the different parts of the plant very clearly. This should be mounted dry. In the same way sections of the leaf-stalks of ferns may also be cut, some of which, as Dr. Carpenter states, show the curious ducts very beautifully, especially when cut rather obliquely.
When sections of the softer substances are required, no instrument can be compared with “Valentin’s knife,” which consists of two steel blades lying parallel with one another and attached at the lower end. The distance of separation may be regulated at will by a small screw near the handle. When, therefore, a section is wanted, the substance must be cut through, and betwixt the blades a thin strip will be found, which may be made of any thickness, according to the distance of their separation. By loosening the screw the blades may be extended, and the section may be floated out in water if the damp will not injure it. The knife cuts much better if dipped in water immediately before use and also when the substance to be operated upon is wet, or even under water altogether; but care must be taken, after use, to clean the blades th............