CHAPTER VI.

ON MOUNTING.

SLIDES.

GLASS slides, 3 x 1, must be cleaned before using, and a good plan is to keep some cleaned, ready for use, in a two-dozen box with rack work, where they stand on their edges and do not get dusty. The ordinary slides sold at the shops at 6s. a gross, are easily cleaned with a chamois leather. Sometimes, however, especially when using slides for the second time, they cannot be cleaned so readily, and they must be soaked in a decoction of oak galls for some hours; this is made by pouring boiling water on bruised oak galls and straining.

COVER GLASSES.

The usual size of these is of an inch square, but larger ones will be required, and some of of an inch should be obtained.

For ordinary work square cover glasses will do, but for objects that have to be sealed up with Hollis's glue, either because they are to be examined at once with an oil immersion or when they are mounted in a watery medium or glycerine, round covers are better, as they can be so easily sealed with a turntable.

They must, however, be mounted in the centre of the slide, and for this purpose what is called a mounting card is used. This is merely a square piece of cardboard with a inch circle and centre dot, while two pieces of cardboard at right angles are glued on to it, so that when the slide is pushed up against them the circle and dot show through where the exact centre is.

The student is cautioned against buying cheap slides and cover glasses as they do not pay, a large number having to be rejected on account of flaws, it is much better to pay a little more and get slides which will not spoil a good preparation.

ON MEASURING COVER GLASSES.

For ordinary students' work the No. 1 cover glasses will do perfectly well, as they are very near the thickness to which object glasses, without a correction collar, are adjusted; but when high power glasses are to be used, it facilitates the work very much, to know the exact thickness of the cover glass under which the specimen is mounted, and with very high powers, or those with wide angles of aperture, the cover must be at least 004 to enable the glass to work through it. Powell and Lealand's water immersion requires a cover glass 003 of an inch.

It is a good plan to measure an ounce of No. 1 cover glasses occasionally, and put by those of thicknesses of 004 and under. This is readily done by a small lever instrument made by Messrs. Stevens &

Sons, Instrument makers, 159 Gower Street, only a few of these very thin glasses will be required for special work, but it is as well to have them in readiness.

ON CLEANING COVER GLASses.

The following plan will be found a very good one, both for saving time and breakage.

Place the cover glasses to be cleaned in a glass vessel containing strong sulphuric acid, and agitate gently until the acid has penetrated between the glasses and driven out the air-bubbles, let them remain in this for an hour or two and then wash well in water until no acid is left. Remove them to a capsule containing methylated spirit. Take out each one separately with a pair of broad pointed forceps, and wipe dry with a silk or soft linen rag.

With very thin cover glasses, such as '003, each glass may be dipped in absolute alcohol when taken out of the methylated spirit and then carefully dried with an old silk handkerchief.

MOUNTING FLUIDS.

For fresh tissues:

Glycerine.

For hardened tissues :

Canada balsam, chloroform and turpentine.

Canada balsam and xylol.

Canada balsam and benzole.

Dammar varnish.

MOUNTING FRESH TISSUES.

Place the tissue to be mounted in a capsule of water of sufficient depth to cover more than half of an ordinary glass slide, when placed in it with one end on the bottom and the other resting on the opposite side. With a needle, bring the tissue over the middle of the slide and hold it there, at the same time raise the upper end of the slide very gently, so that the tissue will adhere to it and be raised out of the water. See that it is not folded in any part. Lay the slide on some filter paper, and with needles spread out the tissue to its fullest extent, without stretching it. It is necessary to be very careful of this, as if the tissue be a serous membrane, stained with silver, the outlines of the cells will be completely destroyed wherever it has been stretched. In the same way, non-striped muscle fibre in the mesentery of the newt, will be broken up and quite ruined.

When the tissue appears to be extended in a natural manner, without folds, take up the slide and wipe off all moisture from it with a clean cloth. If there is a large quantity on the specimen, some may be removed with a bit of filter paper, but great care must be taken not to touch the specimen itself with the paper as it will adhere to it; at the same time it must not be allowed to become dry, and if this seems probable, it can easily be moistened by breathing on it occasionally, until the cover glass is ready. Take up a clean cover glass and place a drop of glycerine on

F

the centre, invert and place it horizontally on the specimen, leaving the weight of the cover glass to spread out the glycerine. If there is an excess of glycerine round the edges of the cover glass, it must be removed by placing small pieces of filter paper in contact, which will soon absorb the superfluous fluid, but must not be left too long or they will drain it from under the cover glass. When the superfluous glycerine has been removed by the pieces of filter paper, take them off and wipe the slide with a dry cloth, taking care not to move the cover glass.

When this is done the preparation must be sealed, by painting round the cover glass with either Dammar varnish or Hollis's glue, taking care that only the extreme edge of the cover glass is included. It will be necessary to give a second and third coat if Dammar varnish is used, at intervals of a few days.

It will be found a good plan to seal first with Dammar varnish, and afterwards to cover this with Hollis's glue, as it makes the preparation more secure, and it is absolutely necessary to have them sealed with Hollis's glue when oil immersion lenses are to be used, as the cedar oil does not touch it, while it dissolves Dammar varnish at once.

Specimens carefully prepared in the above manner may be kept for years without deteriorating.