ITHIN the last few years the art of section-cutting has been greatly improved, but accounts of the methods in use have as yet hardly found their way into ordinary microscopic handbooks, and to the amateur, who modestly restricts his dissections to insects, they are, I believe, almost unknown. The general principles of the methods are in all cases the same, but the application of them to various sorts of tissues is frequently different; so, in working out the details for insects, although I write chiefly for the amateur, I may possibly contribute something of use to the student.

Most microscopists find that a great deal of the interest of their microscopy lies in preparing specimens. If there be any one of these who has never tried section-cutting, let him buy a microtome and set to work at once. It is quite a fascinating amusement, a kind of refined "whittling a stick "-that pleasure so dear to youth.

After a few remarks on microtomes, I propose to divide my observations into four parts; Preparing the specimens ; cutting, mounting, and staining them. I would premise that, if I recommend a process for insects, it does not follow that it will do for anything

élse.

The most important thing in getting perfect sections is the proper preparation of the insect, previous to cutting it. If an object be well prepared, No. 277.-JANUARY 1888.

good sections may be obtained with the commonest of microtomes; of course, the better the microtome, the better the sections: yet the best of microtomes is useless, if the specimens be not rightly prepared. Section machines seem to vary in price from 8s. to £8. As far as I know, the cheapest are practically as good as those of medium price, for when cutting is effected by holding the razor in the hand, one can

never be sure to the th of an inch where its edge will come, in consequence of the elasticity of the steel. This is the arrangement in all the moderate priced instruments I have seen, so that a £2 machine is not essentially better than an 8s. one. In the more expensive forms this uncertainty is eliminated by the motion of the razor (or object) being effected mechanically. The best of these is said to be the Cambridge "Rocking Microtome," price £5 5s. I have got very good sections with a section machine of the cheapest kind, and I have seen most excellent vegetable sections which were cut without a microtome at all. So the student need not lay aside sectioncutting for lack of an expensive machine. A good razor, however, is a necessity.

PREPARING SPECIMENS.

A great many fluids have been compounded for hardening tissues previous to cutting them. Some do very well for insects, but any which contain chromic acid are totally destructive. This reagent renders chitine extremely friable, and in fact, makes the whole section so brittle, that it is hardly possible to mount it unbroken. Moreover, in spite of all said to the contrary, it prevents proper staining. But the fluid readiest to hand is common methylated spirits, and this answers every purpose. Soak the insects in it for a week or two, or as much longer as you like. Soft bodied insects may shrink if put into methylated spirits at once. A day's previous soaking in dilute picro-sulphuric acid solution will prevent this. Then let them have three or four days in absolute alcohol, changing the alcohol once. Transfer them to oil of

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cloves, in which let them stay for a day; and, after the oil of cloves, let them be in turpentine another day. For some reason that I do not understand, soaking first in oil of cloves and then in turpentine prevents tissues from shrinking when they are put into wax. If oil of cloves, turpentine, or chloroform be used alone, shrinking can hardly be avoided. For this " wrinkle " I am indebted to Dr. Schönland, of the Botanic Gardens, Oxford.

Now melt some paraffin wax before a gas-stove or over a water bath. Be very careful to have it only just above its melting-point. Warm the bottle of turpentine containing the specimens, pick them out, and drop them into the melted wax. This must be maintained at the same temperature for about ten hours. Great care must be taken not to let it get too hot, or the insects will shrink. If you put the specimens in at night, they will be ready by morning. By this time the wax should have penetrated every part of the insect, and the excellence of the sections depends upon this being done completely. No perfection of microtome will compensate for imperfectly imbedded specimens ; but, as I have said, you can cut good sections of a well-imbedded insect in a common machine. This method of imbedding answers for most insects, but very soft ones need yet more care to avoid shrinkage. Either of the two following methods will do. Put the insects into a small open vessel with enough turpentine to cover them, add a good many chips of wax. Or this melt some wax in the vessel and let it get cold; put the insects with enough turpentine to cover them on the top of this. Then (in either case) gradually bring the wax to the melting-point before the stove or in a hotair box, and keep it just melted for hours until the turpentine is all evaporated. The penetration of the wax is so gradual, that there is no danger of the insect shrinking.

The vessel in which the wax is allowed to get cold must be flat bottomed; and the objects should be arranged in it at intervals in such positions that the bottom of the vessel is at right angles to the plane of the sections which you intend to cut. When the wax is quite hard, the specimens may be carefully cut out in little cubes of wax, and kept for any length of time until wanted.

The wax used must be pure paraffin. I have tried all sorts of mixtures and different waxes, but nothing does so well as pure paraffin. It is of great importance to have it of the right melting-point. The Cambridge people say that with their microtome this does not matter, but my experience with the machine is different. If the temperature of your room be 56° to 60° Fahr., the proper melting-point of the wax is 110°. You should have a thermometer, and, whenever you want to cut sections, you must bring the temperature to this point. In summer, when you cannot cool the air to 60°, you must use harder wax, preserving the difference of 50° between the tempera

ture of the room and the melting-point of the wax.. The reason for this particularity is that, if the wax be too hard, the sections will curl up as you cut them, and frequently break. On the other hand, if it be too soft, it gives way under the pressure of the razor. Any grocer will get you paraffin through Price's (or some other) candle company. It is made of four degrees of hardness, melting at 110°, 115°, 120°, and 125°. The probable price will be 6d. per pound.

CUTTING THE SECTIONS.

If you have a microtome of the common sort, with a well, fill the well with melted wax, and let it get quite cold. Then screw it up until about half-an-inch of wax appears above the cutting plate of the instrument. Remove this with the razor, and if the block of wax seem at all loose in the well, thrust a thin splinter of lucifer match between the wax and the side of the well. Take now one of the cubes of wax containing an insect to be cut; square it roughly with a pen-knife; fasten it to the wax in the well by means of the heated blade of an old knife. When it is cold and hard again, finish off the squaring process accurately, taking care that the opposite sides are parallel. Screw down the machine until the top of the little cube is level with the surface of the cutting plate, and the specimen is ready for cutting. Take off a few preliminary slices, and when you have cut down to the object, place the razor with its edge parallel to and almost touching one of the sides of the cube. Draw it sharply towards you without any motion sideways, but in a direction exactly at right angles to that side of the block with which its edgeis parallel. Let the section remain on the razor blade, and after again turning up the screw of the microtome, repeat the cut, using exactly the same portion of the razor edge to cut with by placing the section already cut precisely behind the little square block of wax. The new section will stick to the edge of the first one and push it across the razor blade.. The same process, indefinitely repeated, will produce a ribbon of sections almost as neatly as a rocking microtome will do it. Success depends on the melting-point of the wax, and the temperature of the room, being properly adjusted, as before explained.

After a little practice, I could readily get unbroken ribbons of 20 to 40 sections. After this, they became too long to be manageable. The advantages of ribbons over single sections I consider to be two :they are much easier to manipulate; and they facilitate getting a series of sections in their proper order. This is an important point when you are cutting up an insect. Besides, having all the sections. cut before any are put upon the slide, you are able to count them, and so calculate the space they will occupy. You can, therefore, arrange them with proper regard to the middle of the slide-a great thing to all who love neatness.