As the object is to be thoroughly immersed in the balsam, it is evident

that when it has once been covered, so it must remain, unless we again

free it by a process hereafter mentioned, which is very troublesome;

and on this account there must be nothing whatever in the balsam except

the object. The inexperienced may think this an unnecessary caution;

but the greatest difficulty he will meet with is to get rid of minute

bubbles of air, perhaps invisible to the naked eye, which appear

like small globules when under the microscope, and render the slide

unsightly, or even worthless. Ten objects out of eleven contain air, or

at least are full of minute holes which are necessarily filled with it;

so that if they should be immersed in any liquid of thick consistency,

these cells of air would be imprisoned, and become _bubbles_. The air,

then, must be got rid of, and this is usually accomplished by soaking

for some time in turpentine, the period required differing according

to the nature of the object. In some cases, the turpentine acts upon

the colour, or even removes it altogether, so that it must be watched

carefully. Often, however, this is an advantage, as where the structure

alone is wanted, the removal of the colouring matter renders it more

transparent. There are objects, however, which retain the air with such

tenacity that soaking alone will not remove it. If these will bear heat

without being injured, they _may_ be boiled in turpentine, or even in

balsam, when the air will be partly or totally expelled. But where

heat is objectionable, they must be immersed in the turpentine, and so

submitted to the action of the air-pump. Even with this aid, sometimes

days are required to accomplish it perfectly, during which time the air

should be exhausted at intervals of five or six hours, if convenient,

and the objects turned over now and then.

Sometimes the objects are so minute that it is impossible to submit

them to any soaking, and in this case they must be laid upon the slide

at once, and the turpentine applied to them there. But it must not be

forgotten that there are some few which are much better mounted in such

a way that the balsam may thoroughly surround, and yet not _penetrate_,

the substance more than necessary. Sections of teeth are amongst these,

which will be noticed in another place, and some insects (see Dr.

Carpenter) when required to show the “ramifications of the tracheæ.”

Having freed the object, then, from these two enemies--dampness and

air--we must proceed to mount it.

The slide must first be cleaned; then on the centre a quantity of

balsam must be placed with a bluntly-pointed glass rod, according to

the size of the object about to be mounted. To this a slight heat must

be applied, which will cause any bubbles to rise from the surface

of the slide, so that they may be readily removed with a needle.

The object should be freed from all air by steeping in turpentine,

as before described, and then from superfluous liquid by a short

drainage, and carefully laid _upon_, or where it is practicable thrust

_into_, the balsam, prepared on the slide as above. In the former

case, or where the balsam has not totally covered the object, a small

quantity must be taken, warmed, and dropped upon it, and any bubbles

removed by the needle as before. To cover this, the thin glass must be

warmed, and beginning at one side, allowed to fall upon the balsam,

driving a small “wave” before it, and thus expelling any bubbles which

may remain. This is quite as safely performed (if not more so) by

making a solution of balsam in turpentine of the consistency of thick

varnish. The thin glass cover may be slightly coated with this, and

will then be much less liable to imprison any air, which frequently

happens when the cover is dry. Bubbles, however, will sometimes

make their appearance in spite of all care; but when the object is

comparatively strong, they may be removed by keeping the slide rather

warm, and _working_ the cover a little, so as to press them to one

side, when they should be immediately removed with a needle point,

otherwise they are again drawn under.

Where the slide requires keeping warm for any length of time, a

_hot-water bath_ is sometimes made use of, which is simply a flat

tin, or other metal case, with a mouth at the side, that when the hot

water is introduced it may be closed up, and so retain its warmth for

a long time. In working, the slide is laid upon it, and so admits of

longer operations, when required, without growing cold. Sometimes

a spirit-lamp is placed under it to keep up an equal heat through

excessively long processes. Where the time required, however, is but

short, a thick brass plate is sometimes used (see Chapter I.), which is

heated to any degree that is required, and the slide placed upon it.

Some objects, which are so thin that they are usually _floated_ upon

the slide, as before stated, require no steeping in turpentine or other

liquid. These are best mounted by covering with a little _diluted_

balsam, and after this has had time to penetrate the substance,

ordinary balsam is laid upon it, and the slide finished in the usual

manner.

I have stated that the balsam is usually applied to the slide and

objects with a “bluntly-pointed glass rod;” but for the purpose of

drawing the balsam from the bottle, and conveying it to the desired

place, Dr. Carpenter uses a glass syringe with a _free_ opening. These

are his instructions:--“This (the syringe) is most readily filled with

balsam, in the first instance, by drawing out the piston, and pouring

in balsam previously rendered more liquid by gentle warmth; and nothing

else is required to enable the operator at any time to expel precisely

the amount of balsam he may require, than to warm the point of the

syringe, if the balsam should have hardened in it, and to apply a very

gentle heat to the syringe generally, if the piston should not then be

readily pressed down. When a number of balsam objects are being mounted

at one time, the advantage of this plan in regard to facility and

cleanliness (no superfluous balsam being deposited on the slide) will

make itself sensibly felt.”

When the “mounting” is thus far accomplished, the outer “wall” of

balsam may be roughly removed after a few hours have elapsed; but

great care is necessary lest the cover be moved or interfered with in

any way. In this state it may be left for the final cleansing until

the balsam becomes hard, which takes place sooner or later, according

to the degree of warmth it has been subjected to. A mantel-piece, or

some place about equal to it in temperature, is the best suited to

this purpose; and when the requisite hardness is attained, it may be

proceeded with as follows:--With a pointed knife the balsam must be

scraped away, taking care that the thin glass be not cracked by the

point getting _under_ it. If used carefully, the knife will render the

slide almost clean; but any minute portions which still adhere to the

glass must be rubbed with linen dipped in turpentine or spirit. If the

balsam is not very hard, these small fragments are readily removed by

folding a piece of paper tightly in a triangular form with many folds,

and damping the point with which the glass is rubbed. As the paper

becomes worn with the friction, the balsam will be carried off with it.

In some cases I have found this simple expedient very useful.

Sometimes the object to be mounted is of such a thickness as to

require a cell. For this purpose glass rings are used (as described in

Chapter IV.), and filled with balsam. The best mode of doing this is

thus described by Mr. T. S. Ralph in the _Microscopic Journal_:--“The

question was asked me when I was in England, if I knew how to fill a

cell with Canada balsam and leave behind no air-bubbles? I replied in

the negative; but now I can state how to accomplish this. Fill the cell

with clear spirit of turpentine, place the specimen in it, have ready

some balsam just fluid enough to flow out of the bottle when warmed by

the hand; pour this on the object at one end, and, gradually inclining

the slide, allow the spirit of turpentine to flow out on the opposite

side of the cell till it is full of balsam; then take up the cover, and

carefully place upon it a small streak of Canada balsam from one end

to the other. This, if laid on the cell with one edge first, and then

gradually lowered until it lies flat, will drive all the air before

it, and prevent any bubbles from being included in the cell. It can

be easily put on so neatly as to require no cleaning when dry. If the

cover is pressed down too rapidly, the balsam will flow over it, and

require to be cleaned off when hardened, for it cannot be done safely

while fluid at the edges.”

Sometimes with every care bubbles are enclosed in the balsam, injuring

objects which are perhaps rare and valuable. The whole slide must then

be immersed in turpentine until the cover is removed by the solution of

the balsam; and the object must be cleansed by a similar steeping. It

may then be remounted as if new in the manner before described.

The balsam and chloroform described in Chapter I. is thus used; and

where the object is thin, the mounting is very easily accomplished.

When the object is laid upon the slide with a piece of glass upon it,

and the balsam and chloroform placed at the edge of the cover, the

mixture will gradually flow into the space betwixt the glasses until

the object is surrounded by it, and the unoccupied portion filled. The

chloroform will evaporate so quickly that the outer edge will become

hard in a very short time, when it may be cleaned in the ordinary way.

Sometimes the balsam is dissolved in the chloroform without being first

hardened; but this is only to render it more fluid, and so give the

operator less chance of leaving bubbles in the finished slide, as the

thicker the medium is, the more difficult is it to get rid of these

intruders.

It has been before mentioned that some have objected to chloroform

and balsam, believing that it became _clouded_ after a certain time.

Perhaps this may be accounted for in part by the fact that almost all

objects have a certain amount of dampness in them. Others are kept in

some preservative liquid until the time of mounting, and these liquids

generally contain certain salts (Chapter IV.). If this dampness, as

well as all traces of these salts, however small, are not totally

removed--the former by drying, the latter by repeated washings--the

addition of chloroform will render the balsam much more liable to the

cloudiness than when balsam alone was used, as before mentioned.

This mode of employing the balsam, however, will not be always

applicable, as _chloroform_ acts upon some substances which balsam

_alone_ does not. Some salts are even soluble in it, the crystals

disappearing after a few days or weeks, whereas in the balsam alone

they are quite permanent. Experience is the only guide in some cases,

whilst in others a little forethought will be all that is required.

The particular methods used for certain objects may be now entered

upon. Many of the Diatomaceæ and fossil Infusoria, as they are

sometimes termed, are mounted dry, and cleaned in the way described in

Chapter II. Others are almost always placed in balsam, except where

they are intended to be used with the lieberkuhn and dark background,

by which means some of them are rendered exquisitely beautiful. The

usual way of mounting them in balsam is as follows:--Take a drop of

the water containing them, place it upon the slide, and evaporate over

the lamp, whilst with a needle they may be dispersed over any space

desired. When they are thoroughly dry, drop a little balsam on one

side, and exclude the bubbles. The slide may then be warmed to such

a degree that the balsam, by lifting the glass at one end, will be

carried over the specimens, which may then be covered with thin glass,

made warm as before described. Where the objects are quite dry, and

loose upon the glass, it requires great care in placing the cover upon

them, otherwise they are forced to one edge, or altogether from under

it, in the wave of the balsam. For this reason, Professor Williamson

adds a few drops of gum-water to the last washing, which causes them to

adhere sufficiently to the glass to prevent any such mishap.

Mr. T. G. Rylands’ method differs in some degree from the above, and

is, to use his own words, as follows:--Thick balsam is preferable,

and the burnt covers (see Chapter II.) to be mounted are laid in a

convenient position with the diatoms upwards. The slides required

having been carefully cleaned and marked on the under side with a ring

of ink, by the aid of a turntable about half an inch in diameter to

point out the centre, a drop of benzole is applied by a large pin to

the diatoms on the cover, so as to exclude the air from the valves

and frustules. The slide is then held over the lamp, and when warm,

a sufficiently large drop of balsam is put upon it, and heated until

it begins to steam. If small bubbles appear, a puff of breath removes

them. The slide being held slightly inclined from the operator, and the

drop of balsam becoming convex at its lower edge, the cover is brought

in contact with it at that point, gradually laid down, pressed with the

forceps, and brought to its central position. When cool the superfluous

balsam (if any) is removed with a heated knife-blade, the slide cleaned

with a little turpentine, and finished by washing in a hand-basin with

soap and water. In this process there is no delay if the balsam be

sufficiently thick, as the slide may be cleaned off almost before it is

cold.

It is now well known that from common chalk it is an easy matter to

obtain interesting specimens of Foraminifera. Scrape a small quantity

of chalk from the mass and shake it in water; leave this a few minutes,

pour the water away and add a fresh quantity, shake up as before, and

repeat two or three times. Take a little of the residue, and spread it

upon the slide, and when quite dry add a little turpentine. When viewed

with a power of two hundred and fifty diameters this will generally

show the organisms very well. If it is desired to preserve the slides,

they may be then mounted in Canada balsam. Mr. Guyon, in “Recreative

Science,” observes that the accumulation of the powder, by the action

of the rain or exposure to the atmospheric action, at the foot or any

projection of the chalk cliffs, will afford us better specimens than

that which is “scraped,” as the organisms are less broken in the former.

When the Foraminifera are of a larger size, though transparent enough

to be mounted in balsam, the air must be first expelled from the

interior, otherwise the objects will be altogether unsatisfactory. To

accomplish this they must be immersed in turpentine and submitted to

the action of the air-pump. So difficult is it to get rid of this enemy

that it is often necessary to employ three or four exhaustions, leaving

them for some time under each. When all air has given place to the

turpentine, they must be mounted in the ordinary way.

Of all objects which are commonly met with, few are such general

favourites as the POLYCYSTINÆ, and deservedly so. Their forms are

most beautiful, and often peculiar--stars varying in design, others

closely resembling crowns; the _Astromma Aristotelis_ like a cross,

and many whose shapes no words could describe. The greater part,

perhaps, of those which are usually sold, is from the rocky parts of

Bermuda; but they are also found in Sicily, some parts of Africa and

America. They are usually mounted in balsam, but are equally beautiful

mounted “dry” and used with the lieberkuhn. They require as much care

in cleaning as the Diatomaceæ, but the process is a different one.

Sometimes this is effected by simply washing until they are freed from

all extraneous matter, but this is seldom as effectual as it should be.

In the _Microscopic Journal_ Mr. Furlong gives the following method of

treatment as the best he knew:--

Procure--

A large glass vessel with 3 or 4 quarts of water.

New tin saucepan holding 1 pint.

2 thin precipitating glasses holding 10 oz. each.

Take 3 oz. of dry “Barbadoes earth” (lumps are best), and break into

rather small fragments. Put 3 or 4 oz. of common washing soda into the

tin and half fill it with water. Boil strongly, and having thrown in

the earth, boil it for half an hour. Pour nine-tenths of this into the

large glass vessel, and gently crush the remaining lumps with a soft

bristle brush. Add soda and water as before, and boil again; then pour

off the liquid into the large vessel, and repeat until nothing of value

remains. Stir the large vessel with an ivory spatula, let it stand for

three minutes, and pour gently off nine-tenths of the contents, when

the shells will be left, partially freed only, like sand.

2ND PROCESS.--Put common washing soda and water into the tin as before,

and having placed the shells therein, boil for an hour. Transfer to the

large vessel as before, and after allowing it to stand for one minute

pour off. Each washing brings off a kind of “flock,” which seems to be

skins.

3RD PROCESS.--Put the shells in precipitating glass and drain off the

water until not more than ½ oz. remains. Add half a teaspoonful of

bicarbonate of soda, dissolve, and then pour in gently 1 oz. of strong

sulphuric acid. This liberates the “flock,” &c., and leaves the shells

beautifully transparent. Wash well now with water to get rid of all

salts and other soluble matter.

Some of the large shells are destroyed by this method, but none that

are fit for microscopic use. An oblique light shows these objects best.

These are sometimes treated in the manner described in Chapter II.

where the diatoms are spoken of, but many forms are liable to be

injured by this severe process.

It has been before stated that some of the zoophytes may be mounted

dry, and others examined as opaque or transparent objects according to

their substance. They are very interesting when examined in the trough

whilst living, but to preserve many of them for future examination they

must be mounted in some preservative medium. Sometimes this may be one

of the liquids mentioned in Chapter IV., but if possible they should

be kept in balsam, as there is less danger of injury by accident to

this kind of slide. This method of mounting presents some difficulties,

but I think that all agree as to the trustworthiness of Dr. Golding

Bird’s information on the subject, which appeared in the _Microscopic

Journal_. Of this, space forbids me to give more than a condensed

account, but I hope to omit nothing of moment to the reader for whom

these pages are written.

After stating that there are few who are not familiar with these

exquisite forms, and have not regretted the great loss of beauty

they sustain in dying, he informs us that from their so obstinately

retaining air in the cells and tubes when dried, it is hardly

practicable to get rid of it; and they also shrivel up very seriously

in the process of drying. The following plan, however, he has found

almost faultless in their preparation.

To preserve them with extended tentacles, they should be plunged in

cold fresh water, which kills them so quickly that these are not often

retracted. The specimens should be preserved in spirit until there is

leisure to prepare them; if, however, they have been _dried_, they

should be soaked in cold water for a day or two before being submitted

to the following processes:--

1. After selecting perfect specimens of suitable size, immerse them

in water heated to about 120° and place them under the receiver of an

air-pump. Slowly exhaust the air, when bubbles will rise and the water

appear to be in a state of active ebullition. After a few minutes

re-admit the air and again exhaust, repeating the process three or four

times. This will displace the air from most, if not all, of the class.

2. Remove the specimens and allow them to drain upon blotting-paper

for a few seconds; then place them in an earthen vessel fitted with a

cover, and previously heated to about 200°. This heat may be easily

got by placing the vessel for a short time in boiling water, wiping

it immediately before use with a thick cloth. The specimens are then

dropped into this, covered with the lid, and immediately placed under

the receiver of the air-pump, and the air rapidly exhausted. By this

means they are dried completely, and so quickly that the cells have no

time to wrinkle.

3. In an hour or two remove them from the air-pump and drop them into

a vessel of perfectly transparent camphine. This may be quite cold

when the horny, tubular polypidoms, as those of the Sertulariæ, are

used; but should be previously heated to 100° when the calcareous,

cellular Polyzoa are the objects to be preserved. The vessel should be

covered with a watch-glass and placed under the receiver, the air being

exhausted and re-admitted two or three times.

4. The slide which is to receive the specimen should be well cleaned

and warmed so as to allow the balsam to flow freely over it. This

must be applied in good quantity, and air-bubbles removed with the

needle-point. Take the polypidom from the camphine, drain it a little,

and with the forceps immerse it fully in the balsam. The glass to be

laid upon it should be warmed and its surface covered with a thin

layer of balsam, and then lowered gradually upon it, when no bubbles

should be imprisoned. A narrow piece of card-board at each end of the

object for the cover to rest upon, prevents any danger of crushing the

specimen.

This mode of mounting polypidoms, &c., seems to give almost the

complete beauty of the fresh specimens. They are very beautiful objects

when viewed with common light, but much more so when the polarizer is

used (in the manner described a little farther on).

To the above instructions there can be little to add; but I may here

mention that some young students may not be possessed of the air-pump,

and on this account put aside all search for those specimens which need

little looking for at the seaside. Many of these, however, though they

lose some beauty by the ordinary mode of drying, will by steeping for

some time in turpentine not only be freed from the air-bubbles, but

suffer so little contraction that they are a worthy addition to the

cabinet.

Another class of objects is the _spicula_ met with in sponges, &c.

These are often glass-like in appearance and of various shapes; many

are found resembling needles (whence their name); some from the synapta

are anchor-like, whilst others are star-like and of complex and almost

indescribable combinations. As some of these are composed of silex and

are consequently not injured by the use of nitric acid, the animal

substance may be got rid of by boiling them in it. Those, however,

which are calcareous must be treated with a strong solution of potash

instead; but whichever way is used, of course they must afterwards be

freed from every trace of residue by careful washing.

These spicules may be often found amongst the sand which generally

accumulates at the bottom of the jars in which sponges are kept by

those who deal in them, and must be picked out with a camel-hair

pencil. The specimens obtained by this means will seldom if ever

require any cleaning process, as they are quite free from animal

matter, &c.

In the former chapter was noticed those insects or parts of them which

are usually mounted dry. When they are large and too opaque to admit of

the dry treatment, they must be preserved in Canada balsam or fluid.

The first of these may now be considered.

It may be here mentioned, that with these objects much heat must not be

employed, as it would in some instances give rise to a cloudiness, and

almost invariably injure them.

In killing the insect it is necessary not to rub or break any part of

it. This may be performed by placing it in a small box half filled with

fragments of fresh laurel leaves, by immersion in turpentine or strong

spirit, as also in solutions of various poisonous salts. After which

it may be preserved for sometime in turpentine or other preservative

liquid (Chapter IV.) until required. As an assistance to the student,

I believe that I can do no better than give him the plan pursued by my

friend Mr. Hepworth, whose specimens are in every way satisfactory;

but when his method is used, the insects must not have been placed in

turpentine for preservation:--

“After destroying the insects in chloroform or sulphuric ether

(methylated being cheaper), wash them thoroughly in a wide-necked

bottle, half-filled, with two or three waters; the delicate ones

requiring great care. Then immerse them in liquid potash (or Brandish’s

solution, which is stronger than the usual preparation), and let them

remain a longer or shorter time according to their texture. When ready

to remove, put one by one into a small saucer of clear water, and with

a camel-hair pencil in each hand press them flat to the bottom, holding

the head and thorax with the left-hand brush, and apply pressure with

the other from above, downwards, giving the brush a rolling motion,

which generally expels the contents of the abdomen from the thorax.

A minute roller of pith or cork might be used instead of the brush.

In larger objects, use the end of the finger to flatten them. Large

objects require more frequent washing, as it is desirable to remove the

potash thoroughly, or crystals are apt to form after mounting. Having

placed them on the slides with thin glass covers, tied down with

thread,[C] dry and immerse them in rectified spirits of turpentine;

place the vessel under the receiver of an air-pump, and keep it

exhausted until the turpentine has taken the place of the air-bubbles:

they are then ready for the application of the balsam. Larger objects

may often with advantage be transferred to a clean slide, as during the

drying there is considerable contraction, and an outline often remains

beyond the margin showing this. When closely corked they may remain in

the spirits two or three months. As you take them from the bottle, wipe

as much turpentine off as possible before removing the thread, and when

untied carefully wipe again, placing the finger on one end of the cover

whilst you wipe the other, and vice versâ. By this means you remove

as much turpentine from under the cover as is necessary; then drop

the balsam, thinned with chloroform (see Chapter I.), upon the slide,

letting the fluid touch the cover, when it will be taken in between the

surfaces by capillary attraction; and after pressing the cover down it

may be left to dry, or you may hold the slide over a spirit-lamp for

a few seconds before pressing down the cover. If heat is not applied,

they are much longer in drying but are more transparent. If made too

hot the boiling disarranges the objects, and if carried too far will

leave only the resin of the balsam, rendering it so brittle that the

cover is apt to fly off by a fall or any jar producing sufficient

percussion. Never lift the cover up, if possible, during the operation,

as there is danger of admitting air. A few bubbles may appear

immediately after mounting, but generally subside after a few hours,

being only the chloroform or turpentine in a state of vapour, which

becomes condensed.”

[C] This applies to the more delicate ones, which will not bear

transferring after being once spread out and dried.

This method of preparing and mounting insects I can strongly recommend

as giving first-rate results; but where the specimens are small they

seldom need the soaking in caustic potash which larger ones must have.

It is only necessary to leave them awhile in turpentine, especially

when they have been first dried with gentle pressure between two

glasses, and then mount with balsam in the ordinary way.

Amongst the insect tribes there is abundant employment, especially

for the lower powers of the microscope. But if the deeper wonders and

beauties of the animal economy are to be sought out and studied, it is

desirable that the various parts should be set separately, in order

that they may receive a more undivided attention, as well as to render

them capable of being dealt with under the higher powers. We will,

therefore, briefly consider the treatment which the different portions

require.

The eyes of the butterflies, and indeed of almost all insects, afford

materials for a study which is complete in itself. When examined with

a tolerably high power, instead of finding each eye with an unbroken

spherical surface, it is seen that many are composed of thousands of

hexagonal divisions, each being the outer surface of a separate portion

termed the _ocellus_. In others these divisions are square; but in

all there is a layer of dark pigment surrounding their lower parts.

The ocelli may be partly removed from the eye, which will show how

their tapering forms are arranged. But here we have to consider how to

place them in balsam for preservation. The eye being removed from the

insect, and the dark pigment removed by the use of a camel-hair pencil,

must be allowed to remain in turpentine at least for some days. The

turpentine should then be renewed and the eye well washed in it just

before it is to be mounted. It may then be set in balsam in the same

way as any other object;--but here a difficulty is met with. The eye

being spherical upon the surface required, must necessarily be “folded”

or broken in attempting to flatten it. This difficulty may be often

overcome by cutting a number of slits round the edges; but some object

to this mode of treatment, and where it is practicable it is much more

satisfactory to mount one in the natural rounded form and another

flat. Instead, however, of mounting the organ _whole_, four or five

slides may be procured from each of the larger ones, as those of the

dragon-fly, &c.

The _antennæ_ also are often mounted on separate slides, as being

better suited for higher powers and more minute examination than when

connected with the insect. These two projecting organs, issuing from

the head, are jointed, and moveable at will. They differ very much in

form amongst the various species, and are well worth the attention of

the microscopist. They are usually mounted with the head attached, and

perhaps they are more interesting when thus seen. Some few are very

opaque, to prepare which the following method has been advised:--

Bleach the antennæ by soaking in the following solution for a day or

two:--

Hydrochloric acid, 10 drops.

Chlorate of potash, ½ drachm.

Water, 1 oz.

This will render them transparent. Wash well, dry, and mount in Canada

balsam. Instead of the above, a weak solution of chloride of lime may

be used, by which means the nerves will be well shown. Many, however,

are rendered transparent enough by simply soaking in turpentine for

a longer or shorter time. Where the antennæ, however, are “plumose,”

or feather-like, extreme care is required in mounting, though the

difficulty is not so great as some seem to think. If they are first

dried with gentle pressure, and then subjected to the action of the

air-pump in a small quantity of turpentine until the air is thoroughly

expelled, they can be easily finished upon the slide, especially when

balsam and chloroform are used.

Insects supply us with another series of beautiful objects, viz.,

the _feet_.[D] These are sometimes simply dried and mounted without

any medium, as before mentioned; but most of them are rendered much

more fit for examination by using balsam in their preservation, as it

greatly increases their transparency. The smaller kinds may be dried

with gentle pressure betwixt blotting-paper, and then immersed for

some days in turpentine, without requiring the treatment with liquor

potassæ. This immersion will render them beautifully transparent, when

they may be mounted in balsam in the usual manner.

[D] See Mr. Hepworth’s interesting articles on the fly’s

foot in the second and third volumes of the _Microscopic

Journal_.

It is, however, sometimes found difficult to fix the feet when

_expanded_, in which state the interest of the object is greatly

increased. Mr. Ralph recommends the following mode:--“First wash

the feet, while the insect is yet alive, with spirits of wine; then

holding it by a pair of forceps close to the edge of a clean piece of

glass, the insect will lay hold of the upper surface by its foot, then

suddenly drop another small piece of glass over it, so as to retain

the foot expanded, and cut it off with a pair of scissors, tie up and

soak to get rid of air.” Mr. Hepworth says that he never found any

difficulty in expanding the foot on a drop of water or well-wetted

slide, and laying a thin glass cover over it, tying with thread,

drying, and immersing in turpentine.

The mouth, also, with its organs, is an interesting object in

many insects. That of the common fly is often made use of, and is

comparatively easy to prepare. By pressing the head, the tongue (as it

is commonly termed) will be forced to protrude, when it must be secured

by the same means as the foot, and may be subjected to the soaking

in turpentine, and mounted as usual. The honey-bee is, however, very

different in formation, and is well worth another slide; indeed, even

in insects of the same class, the differences are many and interesting.

Another worthy object of study is the _respiration_ of insects, which

is effected by tracheæ or hollow tubes, which generally run through the

body in one or more large trunks, branching out on every side. These

terminate at the surface in openings, which are termed _spiracles_, or

breathing organs. The _tracheæ_ often present the appearance of tubes

constructed by a twisted thread, somewhat resembling the spiral fibres

of some plants. These are very beautiful objects, and are generally

mounted in balsam, for which reason they are mentioned here; but as

they evidently belong to the “dissecting portion,” they will be fully

treated of in another place.

Amongst the parasitic insects a great variety of microscopic subjects

will be found. As these are usually small, they may be killed by

immersion in spirits of turpentine; and if at all opaque, may be

allowed to remain in the liquid until transparent enough, and then

mounted in Canada balsam.

The acarida, or _mites_ and _ticks_, are well known; none, perhaps,

better than those which are so often found upon cheese. Flour,

sugar, figs, and other eatables, are much infested by them; whilst

the diseases called the _itch_ in man, and the _mange_ in animals,

are produced by creatures belonging to this tribe. These animals

are sometimes mounted by simply steeping them in turpentine, and

proceeding as with other insects. The “Micrographic Dictionary” gives

the following directions as to mounting _parts_ of these:--“The parts

of the mouth and the legs, upon which the characters are usually

founded, may be best made out by crushing the animals upon a slide with

a thin glass cover, and washing away the exuding substance with water;

sometimes hot solution of potash is requisite, with the subsequent

addition of acetic acid, and further washing. When afterwards dried

and immersed in Canada balsam, the various parts become beautifully

distinct, and may be permanently preserved.”

Feathers of different kinds of birds are usually mounted in balsam

when required to show much of the structure. This is particularly

interesting when the feathers are small, as they then show the

inner substance, or _pith_, as it may be termed, with the cells,

&c. The “pinnæ,” or soft branches of the feathers, will be found

of various constructions; some possessing hooks along one side,

whereby they fasten themselves to their neighbours; others branching

out, with straight points somewhat resembling the hairs from certain

caterpillars. But, of course, when the metallic-looking gorgeous

colours are all that is required to be shown, and reflected light used

(as with the feathers of the hummingbird, peacock, &c.), it is much

better that they should be mounted dry, as in Chapter II.

The _seeds_ and _pollen_ of plants are most frequently mounted dry,

as mentioned in Chapter II.; but the more transparent of the former,

and the darker kinds of the latter, are perhaps better seen in Canada

balsam. There is nothing particular to be observed in the manipulation,

except that the glass cover must be applied lightly, otherwise the

grains may be crushed. There are some objects which cannot be shown

in a perfect manner when mounted _dry_, but when immersed in balsam

become so very transparent that they are almost useless. To avoid this,

it has been recommended to stain the objects any colour that may be

convenient, and afterwards mount in balsam in the ordinary manner.

Most objects intended for the polariscope may be mounted in Canada

balsam; but there are some exceptions to this. Many of the salts are

soluble in this medium, or their forms so injured by it, that glycerine

or oil has to be used (see Chapter IV.); others must be left in the dry

form, as before mentioned; and some few it is impossible to preserve

unchanged for any length of time. _Crystals_, however, are amongst the

most beautiful and interesting subjects for polarisation; and it is

very probable that, by the aid of the polariscope, new and valuable

facts are yet to be made known. For one who finds pleasure in form

and colour, there is a field here which will only open wider upon him

as he advances; and instead of being in anywise a merely mechanical

occupation, it requires deep and careful study. The little here said on

the subject will show this in some degree.

With almost every salt the method of _crystallization_ must be modified

to obtain the best forms; I may even go further than this, and say that

it is possible to change these forms to such a degree that the eye

can perceive no relationship to exist betwixt them. If a solution of

sulphate of iron is made, a small quantity spread evenly upon a slide,

and then suffered to dry whilst in a flat position, the crystals often

resemble the fronds of the common fern in shape. But if, whilst the

liquid is evaporating, it is kept in motion by stirring with a thin

glass rod, the crystals form separately, each rhombic prism having its

angles well defined, and giving beautiful colours with the polarized

light. Again, pyro-gallic acid, when allowed to flow evenly over the

slide in a saturated solution, covers the surface in long “needles,”

which are richly coloured by polarized light; but if any small portion

of dust or other matter should form a nucleus around which these

“needles” may gather, the beauty is wonderfully increased. A form very

closely resembling the “eye” of the peacock’s tail, both in form and

colour, is then produced, which to one uninitiated in crystallography

bears very little resemblance to the original crystal. From these

simple facts it will be clearly seen that in this, as in every other

department, study and experience are needful to give the best results.

To obtain anything like uniformity in the formation of crystals upon

the glass slide, every trace of grease must be removed by cleaning with

liquor potassæ or ammonia immediately before using, care also being

taken that none of the agent is left upon the slide, otherwise it may

interrupt and change their relative position, and even their form.

Amongst those which are generally esteemed, the most beautiful are the

crystals of oxalurate of ammonia. The preparation of this salt from

uric acid and ammonia is a rather difficult process, and will not on

that account be described here; but when possessed, a small quantity

of a strong solution in water must be made, and a little placed

on the slide, and evaporated slowly. Part of the salt will then be

deposited in circles with the needle-like crystals extending from

common centres. They should then be mounted in pure Canada balsam;

and, when the best colours are wanted, used with the selenite plate.

Of this class of crystal salicine is a universal favourite, and can be

easily procured of most chemists. The crystals may be produced in two

ways:--A small portion of the salt must be placed upon the slide, and a

strong heat applied underneath until fusion ensues; the matter should

then be evenly and thinly spread over the surface. In a short time

the crystals will form, and are generally larger than those procured

by the following process; but the uncertainty is increased a little

when fusion is used, which, however, is desirable with many salts.

Secondly, make a saturated solution of salicine, which in cold water

is effected by adding one part of the salt to eighteen parts of water.

Lay a little upon the slide, and allow it to evaporate spontaneously,

or with the aid of gentle heat. The crystals are generally uniform, and

with ordinary powers quite large enough to afford a beautiful object.

The circular shape and gorgeous colours of this crystal have made it so

great a favourite that there are few cabinets without it.

Many new forms may be procured by uniting two totally different salts

in solution in certain proportions. This is a field affording new

facts and beauties; but requires some chemical knowledge and much

perseverance to obtain very valuable results. One of the most beautiful

I have met with has been composed of sulphate of copper and sulphate of

magnesia. The flower-like forms and uniformity of crystallization when

successful make it well worth a few failures at first; and as I became

acquainted with some new facts in my frequent trials, I will give the

preparation of the double salt from the beginning.

Make a saturated solution of the two sulphates, combined in the

proportion of three parts copper to one part magnesia, and then add

to the solution one-tenth of pure water. Dust or other impurities

should be guarded against, and the slide made free from all trace

of grease by cleaning immediately before use with liquor potassæ or

ammonia. A drop of the solution should then be placed upon the slide,

and by a thin glass rod spread evenly upon the surface. Heat this

whilst in a horizontal position until the salt remains as a viscous

transparent substance, which will not be effected until it is raised

to a high degree. The slide may now be allowed to cool, and when this

is accomplished, the flower-like crystals will be perceived forming

here and there upon the plate. When these are at any stage in which it

is wished to preserve them, a few seconds’ exposure to the fire, as

warm as the hand can comfortably bear, will stop the expansion, when

the portion which we wish to mount should be cut off from the mass of

salt by simply scratching the film around, and pure Canada balsam with

the thin glass used. Breathing upon the film, or allowing the slide to

become cold and attract the moisture from the atmosphere, will cause

the crystallization to extend, and sometimes greatly rob the effect; so

it is necessary to mount quickly when the desired forms are obtained.

As the crystals are very uncertain as to the place of their formation,

I may here mention that they may be got in _any_ part of the slide

by piercing the film with a needle-point; but in some degree this

necessarily interferes with the centre. As the cause of this has no

need to be entered into here, and has been elsewhere discussed, I shall

only give the above directions, and say that there is a great field in

this branch of study which the microscope alone has opened.

It would be useless to enter into particulars respecting the various

salts and treatment they require, as a great difference is effected

even by the strength of the solution. There are some crystals, also,

which are called forth in insulated portions, showing no formation

upon the ground; but even when mounted in any preserving fluid, and

unchanged for a year, a new action seems to arise, and a groundwork

is produced which bears little resemblance to the original crystal.

Sometimes this new formation adds to the beauty of the slide; in other

cases the reverse is the result, the slide being rendered almost

worthless. This action, I believe, frequently arises from some liquid

being contained in the balsam or other mounting medium used; and this

is rendered the more probable by the crystallization being called

forth in an hour after the balsam diluted with chloroform is employed,

whereas no change would have taken place for months (if at all) had

pure balsam been used.

Sections of some of the salts are very interesting objects, but the

method of procuring these and their nature will be described in Chapter

V.

The scales of various fish have been before mentioned as mounted “dry;”

when, however, they are required for polarising objects they are

generally mounted in balsam, and some few in liquid. The former method

will be considered here.

The eel affords a beautiful object for this purpose. The scales are

covered by a thin “_skin_,” which may be slightly raised with a knife

and then torn off, in the same manner as the covering of the geranium

and other petals, described in Chapter II. The required portion may

then be removed; or if a piece of skin can be procured as stripped off

in cooking, the scales may be easily taken from the inner surface. They

must then be washed and thoroughly cleaned. After drying, soak for a

day in turpentine, and mount in the ordinary manner with balsam. This

is a good polarising object; but the interest, and I think the beauty,

is increased by procuring a piece of eel’s skin with the scales _in

sitû_, washing and drying under pressure, and mounting in balsam as

before. The arrangement of the scales produces beautiful “waves” of

colour, which are quite soothing to the eye after examining some of the

very gorgeous salts, &c.

There are many scales of fish which are good subjects for the

polariscope when mounted in balsam; but as they require no particular

treatment, they need no mention by name.

Among hairs we find some which are beautiful when mounted in balsam

and examined by polarized light. Some, when wanted as common objects,

are always used dry, as before mentioned; but if they are intended to

be shown as _polarizing_ objects, they must be placed in some medium.

The “Micrographic Dictionary” mentions a mode of making an interesting

object by plaiting two series of white horse-hairs at an angle,

mounting in balsam, and using with the polariscope. All hairs, however,

must be steeped in turpentine for a short time before mounting, as they

will thus be rendered cleaner and more transparent. When this is done,

there is no difficulty in mounting them.

Many of the “tongues” of fresh-water and marine mollusca are deeply

interesting and most beautiful objects when examined by polarized

light. As these are usually mounted in balsam, I mention them in this

place; but as they must be removed from the animals by dissection,

particulars respecting them will not be entered into until we come to

the part in which that operation is described (Chapter V.).

The manner of preparing and mounting many of the Polyzoa and Zoophytes

has been before described; but any notice of _polarizing_ objects would

be incomplete without some allusion to them. A small piece of the

_Flustra avicularis_, well prepared, is beautiful when examined in this

manner. No selenite is needed, and yet the colours are truly gorgeous.

It is often met with upon shells and zoophytes of a large size, and

will well repay the trouble of searching for. Many of the Sertularidæ

are very beautiful with polarized light, and, indeed, no ramble upon

the seaside need be fruitless in this direction.

The different _starches_ are quite a study in themselves, and

are peculiarly connected with polarized light. They are found in

the cellular tissue of almost every plant in small white grains

which vary considerably in size; that from the potato averages

one-three-hundredth of an inch in diameter, and that from arrow-root

about one-six-hundredth. To procure starch from any plant, the texture

must first be broken up or ground coarsely; the mass of matter must be

then well washed in gently-flowing water, and, as all starch is totally

insoluble in cold water, the grains are carried off by the current

and deposited where this is stayed. In procuring it from the potato,

as well as many other vegetables, it is but necessary to reduce the

substance to a coarse pulp by the aid of a culinary “grater;” the pulp

should then be well agitated in water, and allowed to rest a short

time, when the starch will be found at the bottom, its lighter colour

rendering it easily distinguishable from the pulp. It should, however,

be washed through two or three waters to render it perfectly clean.

These grains have no crystalline structure, but present a very peculiar

appearance when examined with polarized light. Each grain shows a dark

cross whose lines meet at the point where it was attached to the plant,

called the _hilum_. Round the grain, also, a series of lines are seen,

as though it were put together in plates. This is more distinctly

visible in some kinds than others.

As to the mounting of these starches there is little to be said. If the

grains are laid upon the slide, and as small a portion as possible of

the balsam diluted with turpentine, as before mentioned, be applied,

they will cling to the glass and allow the pure balsam to flow readily

over them without being so liable to imprison air-bubbles when the thin

glass is put upon them.

The raphides, which were fully described in Chapter II., when required

for use with polarized light, must be mounted in balsam, and many are

found which give beautiful colours. They require no peculiar treatment,

but must be washed quite clean before putting up.

There is one class of objects for the polariscope which differs in

preparation from any we have yet considered, and affords very beautiful

specimens. Some of the plants, including many of the grasses and

the Equisetaceæ (_i. e._ horsetails), contain so large a quantity

of silica, that when the vegetable and other perishable parts are

removed, a skeleton of wonderful perfection remains. This skeleton

must be mounted in balsam, the method of performing which will now be

considered.

Sometimes the cuticle of the equisetum is removed from the plant,

others dry the stem under pressure, whilst the grasses, of course,

require no preparation. The vegetable should be immersed in strong

nitric acid and boiled for a short time; an effervescence will go on

as the alkalies are being removed, and when this has ceased more acid

should be added. At this point the modes of treatment differ; some

remove the object from the acid and wash, and having dried, burn it

upon thin glass until all appears _white_, when it must be carefully

mounted in balsam. I think, however, it is better to leave it in strong

acid until all the substance, except the required portion, is removed;

but this will take a length of time, varying according to the mass,

&c., of the plant. Of course, when this latter method is used, the

skeleton must be washed from the acid, &c., before being mounted in

balsam.

These _siliceous cuticles_ are readily found. The _straws_ of most of

the cereals, wheat, oat, &c.; the _husks_, also, of some of these; many

_canes_; the equisetum, as before described; and some of the grasses.

Many of these are everywhere procurable, so that the student can never

want material for a splendid object for the polariscope.

In Chapter II. the _scales_ (or hairs) which are often found upon the

leaves of plants were mentioned as beautiful objects when mounted dry;

but some of these when detached from the leaf--which is easily done

by gently scraping it, when dried, with a knife--present brilliant

starlike and other forms, if mounted in balsam and used with the

polariscope. There is a little danger, when placing the thin glass

upon the balsam, of forcing out the scales in the wave of matter which

is always ejected; this may be overcome by applying to the slide,

previously to placing the objects upon it, an extremely thin covering

of the balsam diluted with turpentine as before mentioned, and thus

giving them every chance of adherence; or by using the balsam with

chloroform, as before noticed. These scales are much more abundant than

was formerly supposed, and daily new specimens are discovered; so that

the student should always be on the lookout for them in his researches

in the vegetable world.

Most classes of objects, and the treatment they require when mounting

them in balsam, have now been considered. The next chapter will be

devoted to the preservative liquids, and the best methods of using

them.

CHAPTER IV.

PRESERVATIVE LIQUIDS, ETC., PARTICULARLY WHERE CELLS ARE USED.

There are many objects which would lose all their distinctive

peculiarities if allowed to become dry, especially those belonging

to the fresh-water Algæ, many animal tissues, and most of the very

delicate animal and vegetable substances in which structure is to be

shown. These must be preserved by immersion in some fluid; but it is

evident that the fluid must be suited to the kind of matter which it

is intended to preserve. As it often requires much study and trouble

to _obtain_ microscopic objects of this class, it is well that their

_preservation_ should be rendered as perfect as possible; and for this

reason the CELLS, or receptacles of the fluids, should be so closed

that all possibility of escape should be prevented. The accomplishment

of this is not so easy a matter as it might appear to the inexperienced.

Before giving any directions as to the manipulation required in

mounting the objects, we must consider the different _liquids_ and

_cells_ which are requisite for their preservation. Of the former there

are a great number, of which the principal may be mentioned.

DISTILLED WATER is strongly recommended by many for Diatomaceæ and

other Protophytes. It has been, however, stated that confervoid growths

often disturb the clearness of the liquid, and on this account various

additions are made to it. A lump of camphor is often left in the

bottle, so that the water may dissolve as much as possible. One grain

of bay-salt and one of alum are added to each ounce of water; or a drop

or two of creosote shaken up with the ounce of water, which should be

afterwards filtered. These additions are often made; perhaps each of

them good for certain objects.

GLYCERINE.--Some affirm this to be one of the best preservative

liquids, especially for vegetable objects; but others think that it is

much better when diluted with two parts of camphor-water, prepared as

above.[E]

[E] Dr. Carpenter says:--“Glycerine has a solvent power for

carbonate of lime, and should not be employed when the

object contains any calcareous structure. In ignorance

of this fact, the author (Dr. C.) employed glycerine to

preserve a number of remarkably fine specimens of the

pentacrinoid larva of the Comatula, whose colours he was

anxious to retain; and was extremely vexed to find, when

about to mount them, that their calcareous skeletons had so

entirely disappeared, that the specimens were completely

ruined.”

GLYCERINE AND GUM.--This is also believed to be a very good liquid for

vegetable tissues, and is thus prepared:--

Pure gum-arabic 1 oz.

Glycerine 1 ”

Water (distilled) 1 ”

Arsenious acid 1½ grains.

Dissolve the arsenious acid in the cold water, then the gum, add the

glycerine, and mix without bubbles.

DEANE’S COMPOUND.--This is usually deemed about the best medium for

preserving Algæ, mosses, &c., and is thus prepared:--Soak 1 oz. of best

gelatine in 4 oz. of water until the gelatine becomes soft, when 5 oz.

of honey heated to boiling-point are added; boil the mixture, and when

it has cooled, but not enough to become stiff, add ½ oz. rectified

spirit with which 5 or 6 drops of creosote have been well mixed, and

filter the whole through fine flannel. This compound when cold forms a

stiff jelly, the use of which will be described elsewhere.

GLYCERINE JELLY.--This mixture closely resembles the above, but as the

composition differs a little it may be mentioned here. It is strongly

recommended by Mr. Lawrance in the _Microscopic Journal_, where he

states “that the beautiful green of some mosses mounted two years

ago, is still as fresh as the day they were gathered;” and that this

is the only medium he knows which will preserve the natural colour of

vegetable substances. He takes a quantity of Nelson’s gelatine, soaks

it for two or three hours in cold water, pours off the superfluous

water, and heats the soaked gelatine until melted. To each fluid ounce

of the gelatine, _whilst it is fluid but cool_, he adds a fluid drachm

of the white of an egg. He then boils this until the albumen coagulates

and the gelatine is quite clear, when it is to be filtered through fine

flannel, and to each ounce of the clarified solution add 6 drachms of a

mixture composed one part of glycerine to two parts of camphor-water.

GOADBY’S FLUID.--This is much used in the preservation of animal

objects; and seldom, if ever, acts upon the colours. It is thus

prepared--Bay salt, 4 oz.; alum, 2 oz.; corrosive-sublimate, 4 grains.

Dissolve these in two quarts of boiling water and filter. For delicate

preparations some recommend that this mixture be reduced by the

addition of an equal quantity of water; but where there is bone or

shell in the object the above acts injuriously upon it, in which case

this fluid may be used:--Bay-salt 8 oz., corrosive sublimate 2 grains,

water 1 quart.

THWAITES’ LIQUID.--This is recommended for the preservation of

Algæ, &c., as having little or no action on the colour, and is thus

prepared:--Take one part of rectified spirit, add drops of creosote

enough to saturate it; to this add sixteen parts of distilled water and

a little prepared chalk, and filter. When filtered, mix with an equal

quantity of camphor-water (as before mentioned), and strain through

fine muslin before using.

CHLORIDE OF ZINC SOLUTION.--In the “Micrographic Dictionary” this is

stated to be “perhaps the best preservative known for animal tissues.”

Persons of great experience, however, have given a very different

opinion; but it is certainly very useful in many cases where a small

degree of coagulating action is not injurious. It is used of strengths

varying according to the softness of the parts to be preserved; the

average being 20 grains of the fused chloride to 1 oz. of distilled

water. To keep this liquid a lump of camphor may be left floating in

the bottle. I have heard complaints that this mixture becomes turbid

with keeping, but I think this must only be the case when some impurity

has got into the bottle.

CARBOLIC ACID.--This substance has not been known long enough to

warrant any decided opinion upon its merits. A solution of 1 part of

acid to 20 of water has been recommended on account of its antiseptic

properties.

CASTOR OIL.--This is a very useful preservative for crystals and

other objects. Many salts are quite destroyed when Canada balsam is

used with them; but very few are acted upon by this oil. To use it,

it must be dropped in a sufficient quantity to cover the crystal or

object to be preserved with a thin coating of oil. It may be necessary

sometimes to spread it with a needle or other instrument. The thin

glass should then be carefully placed upon it, so that all air may

be excluded; and should any oil be forced out, owing to the quantity

used being too great, it must be removed with blotting paper. When

the edge of the thin glass cover and the surrounding parts of the

slide are as clean as possible, a coating of sealing-wax varnish or

liquid glue must be applied and allowed to dry. A second or even a

third coating may be required, but not before the previous cover is

quite dry. These varnishes, however, are very brittle, and it is much

safer, as a finish, to use one of the tougher cements--gold-size, for

instance--which will render it doubly secure.

The above are the principal liquids, &c., used for preserving objects

in cells. The different cells may be here mentioned; and it is

recommended that these should always be kept some time before use in

order that the cement may become perfectly dry; and care must be taken

that no cement be used on which the preservative liquid employed has

any action whatever.

CEMENT CELLS.--Where the object is not very thick, this kind of cell

is generally used. They are easily made with the turntable before

described; but when the objects to be preserved are _very_ minute,

these cells need not be much deeper than the _ordinary_ circle of

cement on the slide. When, however, a comparatively great depth is

required, it is sometimes necessary to make the wall of the cell as

deep as possible, then allow it to dry and make another addition. Of

these cements gold-size is one of the most trustworthy, and may be

readily used for the shallow cells. The asphaltum and india-rubber,

before noticed, I have found very durable when well baked, and

exceedingly pleasant to work with. It may be used of such a thickness

as to give space for tolerably large objects. Black japan also is much

used. Many cements, however, which are recommended by some writers,

are worse than useless, owing to the brittleness which renders their

durability uncertain, as sealing-wax varnish, liquid-glue, &c.

The student may feel himself at a loss in choosing the cement which

will give him the _safest_ cells, many of them becoming partially

or wholly dry in a year or two, as stated in another place. I can

only give him a few general directions, and he must then use his own

judgment. Of course it would be lost labour to employ any cement upon

which the preservative liquid has _any action whatever_. It is also a

good rule to avoid those in whose composition there are any particles

which do not become a thorough and intimate portion, as these unreduced

fragments will almost certainly, sooner or later, prepare a road by

which the liquid will escape; and, lastly, whatever cement he uses, the

cells are always better when they have been kept a short time before

use, as already stated.

GUTTA-PERCHA RINGS have been recommended by some, as affording every

facility for the manufacture of cells for liquids; but they cannot be

recommended, as, after a certain length of time, they become so brittle

as to afford no safeguard against ordinary accidents.

Often the cells must necessarily be of a large size, and for this

reason are made by taking four strips of glass of the thickness and

depth required, and grinding the places where these are to meet with

emery, so as to form a slightly roughened but flat edge. The glass

strip must also be ground on the side where it meets the plate, and

each piece cemented with the marine glue mentioned in Chapter I. in the

following manner:--On that part of the glass to which another piece

is to be attached should be laid thin strips of the glue; both pieces

must then be heated upon a small brass table, with the aid of the

spirit-lamp, until the strips become melted; the small piece is then

to be taken up and placed upon the spot to which it is to be attached,

and so on until the cell is completed. It will be found necessary to

spread the glue over the surface required with a needle or some other

instrument, so that an unbroken line may be presented to the wall of

the cell, and no bubbles formed. Too great a heat will “burn” the

marine-glue, and render it brittle; care must be therefore taken to

avoid this.

When shallow cells are required, those which are made by grinding

a concavity in the middle of an ordinary slide will be found very

convenient. The concavities are cut both circular and oblong; and the

surface being flat, the cover is easily fastened upon it. These are now

cheap, and are very safe as to leakage.

Circular cells with a flat bottom used to be made by drilling a hole

through glass of the required thickness, and fixing this upon an

ordinary slide with marine-glue; but the danger of breakage and the

labour were so great that this method is seldom used now, and, indeed,

the rings about to be mentioned do away with all necessity of it.

GLASS RINGS.--Where any depth is required, no method of making a cell

for liquids is so convenient as the use of glass rings, which are now

easily and cheaply procurable. They are made of almost every size and

depth, and, except in very extraordinary cases, the necessity for

building cells is completely done away with. These rings have both

edges left roughened, and consequently adhere very well to the slide,

this adherence being generally accomplished by the aid of marine glue,

as before noticed with the glass cells. Gold-size has been occasionally

used for this purpose; and the adherence, even with liquid in the

cell, I have always found to be perfect. This method has the advantage

of requiring no heat, but the gold-size must be perfectly dry, and

the ring must have been fixed upon the slide some time before use.

Canada balsam has also been used for the same purpose, but cannot be

recommended, as when it is perfectly dry it becomes so brittle as to

bear no shock to which the slide may be ordinarily exposed.

These are the cells which are mostly used in this branch of microscopic

mounting. The mode of using them, and the different treatment

which certain objects require when intended to be preserved in the

before-mentioned liquids, may now be inquired into.

I may mention, however, that this class of objects is looked upon by

many with great mistrust, owing to the danger there is of bubbles

arising in the cells after the mounting has been completed, even for

years. I know some excellent microscopists who exclude all objects

in cells and preservative liquids from their cabinets, because they

say that eventually almost all become dry and worthless; and this is

no matter of surprise, for many of them do really become so. Perhaps

this is owing to the slides being sold before they could possibly be

thoroughly dry. As to the air-bubbles, I shall have something to say

presently.

We will now suppose the cell employed is made by placing a glass

ring upon the slide with marine glue or gold-size, and is quite dry.

Around the edge of the cleaned thin glass which is to cover it, I

trace with a camel-hair pencil a ring of gold-size, and also around

the edge of the cell to which it is to adhere. Dr. Carpenter objects

to this, as rendering the later applications of the gold-size liable

to “run in.” All danger of this, however, is completely done away with

by leaving the slide and cover for awhile until the cement becomes

partially “fixed,” but still adhesive enough to perform its function

(Chapter II.). With many slides this is not accomplished in less than

twenty-four hours, even if left two or three days no injury whatever

ensues; but with other kinds an hour is too long to leave the exposed

cement, so that the operator must use his own discretion. The liquid

required may be drawn up by the mouth into the pointed tube mentioned

in Chapter I., and then transferred to the cell. In the various books

of instruction, the object is now to be placed in the cell; this,

however, I think a great mistake, as another process is absolutely

necessary before we advance so far. The cell, full of liquid, must

be placed under the receiver of an air-pump, and the air withdrawn.

Almost immediately it will be perceived that the bottom and sides of

the cell are covered with minute bubbles, which are formed by the air

that is held in suspension by the liquid. The slide may now be removed,

and the bubbles may require the aid of a needle or other point to

displace them, so obstinately do they adhere to the surface of the

glass. This process may be then repeated, and one cause, at least, of

the appearance of bubbles in cells of liquid will be got rid of. The

object to be mounted should also be soaked in one or two changes of the

preservative liquid employed, and, during the soaking, be placed under

the air-pump and exhausted. It may then be transferred to the cell,

which will probably cause the liquid to overflow a little. The cover

with the gold size applied to the edge must then be carefully laid upon

the cell, and slightly pressed down, so that all air-bubbles may be

displaced. The two portions of gold-size will now be found to adhere

wherever the liquid does not remain, although the whole ring may have

been previously wet. The outer edge of the thin glass and cell must

now be perfectly dried, and a coating of gold-size applied. When this

is dry, the process must be repeated until the cement has body enough

to protect the cell from all danger of leakage. When some preservative

liquids are used, a scum is frequently found upon the surface when

placed in the cell, which must be removed immediately before the cover

is laid upon it.

I believe this method to be perfectly secure against leakage when

carefully performed; and some of my friends have told me that their

experience (some years) has been equally satisfactory.

In using some of the particular kinds of preservative liquids, it will

be found necessary to make slight change in the manipulation. This will

be best explained by mentioning a few objects, and the treatment they

require.

For the preservation of the Mosses, Algæ, &c., Deane’s compound is

much used, and considered one of the best media. The specimen to be

mounted should be immersed in the compound, which must be kept fluid

by the vessel containing it being placed in hot water. In this state

the whole should be submitted to the action of the air-pump, as it is

not an easy matter to get rid of the bubbles which form in and around

the objects. The cell and slide must be warmed; and heat will also

be necessary to render the gelatine, &c., fluid enough to flow from

the stock-bottle. The cell may then be filled with the compound, and

the specimen immersed in it. A thin glass cover must then be warmed,

or gently breathed upon, and gradually lowered upon the cell, taking

care, as with all liquids, that no bubbles are formed by the operation.

The cover may be fixed by the aid of gold-size, Japan, or any of the

usual varnishes, care being taken, as before, that all the compound is

removed from the parts to which the varnish is intended to adhere.

The glycerine jelly of Mr. Lawrance, before mentioned, requires almost

a similar treatment. “The objects to be mounted in this medium should

be immersed for some time in a mixture of equal parts of glycerine

and dilute alcohol (six of water to one of alcohol). The bottle of

glycerine jelly must be placed in a cup of hot water until liquefied,

when it must be used like Canada balsam, except that it requires less

heat. A ring of asphaltum varnish round the thin glass cover completes

the mounting.”

The Infusoria (see Chapter III.) are sometimes preserved in liquid;

but present many difficulties to the student. Different kinds require

different treatment, and consequently it is well, when practicable, to

mount similar objects in two or more liquids. Some are best preserved

in a strong solution of chloride of calcium, others in Thwaites’

liquid, whilst a few keep their colour most perfectly when in glycerine

alone. Many of them, however, are so very transparent that they present

but faint objects for ordinary observance. The Desmidiaceæ require

somewhat similar treatment, and may be mentioned here. The solution of

chloride of calcium has been strongly recommended; but no preservative

liquid seems to be without some action upon them. Both of the above

classes of objects should be mounted in shallow cells, so as to allow

as high a microscopic power as possible to be used with them.

Many of the ZOOPHYTES which are obtained on our sea-coasts are well

preserved by mounting in cells, in the manner before mentioned, with

Goadby’s fluid, or distilled water with one of the additions noticed

amongst the preservative liquids. For examination by polarized light,

however, they are usually mounted in balsam (see Chapter III.), whilst

those in cells present a more natural appearance as to position, &c.,

for common study.

As to the use of preservative liquids with the Diatomaceæ there are

various opinions. Some experienced microscopists say that there is

little or no satisfaction in mounting them in this way. Dr. Carpenter,

however, explains this difference by his instructions as to what method

should be used when certain ends are desired. He says: “If they can

be obtained quite fresh, and it be desired that they should exhibit

as closely as possible the appearance presented by the living plants,

they should be put up in distilled water within cement-cells; but if

they are not thus mounted within a short time after they have been

gathered, about a sixth part of alcohol should be added to the water.

If it be desired to exhibit the stipitate forms in their natural

parasitism upon other aquatic plants, the entire mass may be mounted

in Deane’s gelatine in a deeper cell; and such a preparation is a very

beautiful object for the black-ground illumination. If, on the other

hand, the minute structure of the siliceous envelopes is the feature

to be brought into view, the fresh diatoms must be boiled in nitric or

hydrochloric acid” (which process is fully described in Chapter II.).

It is very convenient to have many of these objects mounted by two

or more of the above methods; and if they are to be studied, this is

indispensable. Mr. Hepworth once showed me about one hundred slides

which he had mounted in various ways, for no other purpose than the

study of the fly’s foot.

My friend, Mr. Rylands, successfully mounts the diatoms in the state

in which he finds them, and gave me the following method as that which

he always employs. He says that he has had no failures, and hitherto

has found his specimens unchanged. Take a shallow ring cell of asphalt

or black varnish (which must be at least three weeks old), and on the

cell, whilst revolving, add a ring of benzole and gold-size mixed in

equal proportions. In a minute or two pure distilled water is put in

the cell until the surface is slightly convex. The object having been

already floated on to the cover (the vessel used for this purpose being

an ordinary indian-ink palette), is now inverted and laid carefully

upon the water in the cell. By these means the object may be laid down

without being removed. The superfluous moisture must not be ejected by

pressure, but a wetted camel-hair pencil, the size made in an ordinary

quill, being partially dried by drawing through the lips, must be

used repeatedly to absorb it, which the pencil will draw by capillary

attraction as it is very slowly turned round. When the cover comes in

contact with the benzole and gold-size ring, there is no longer any

fear of the object being removed, and a slight pressure with the end

of the cedar stick of the pencil will render the adhesion complete,

and cement the cover closely and firmly to the cell. When dry, an outer

ring of asphalt makes the mounting neat and complete.

The _Fungi_ have been before mentioned; but it may be here stated that

some few of the minute forms are best preserved in a very shallow cell

of liquid. For this purpose creosote-water may be advantageously used.

The _antennæ_ of insects have been before noticed as being very

beautiful when mounted in balsam. This is readily accomplished when

they are large; but those of the most minute insects are much more

difficult to deal with, and are less liable to injury when put up in

fluid. _Goadby’s Fluid_ serves this purpose very well; but, of course,

the object must be thoroughly steeped in the liquid before it is

mounted, for a longer or shorter time according to the thickness.

The _eggs of insects_ afford some worthy objects for the microscope,

amongst which may be mentioned those of the common cabbage butterflies

(small and great), the meadow-brown, the puss-moth, the tortoiseshell

butterfly, the bug, the cow-dung fly, &c. These, however, shrivel up on

becoming dry, and must, therefore, be preserved in some of the fluids

before mentioned. To accomplish this no particular directions are

required; but the soaking in the liquid about to be employed, &c., must

be attended to as with other objects.

_Glycerine_ may be advantageously used for the preservation of various

insects. These should first be cleaned with alcohol to get rid of all

extraneous matter, and then, after soaking in glycerine, be mounted

with it like other objects. This liquid may also be used for the

Entomostraca, which offer a wide field for study. They are to be

found abundantly in ponds of stagnant water, &c. Some insects, such

as May-flies, &c., are, however, often preserved by immersion in a

solution of one part of chloride of calcium in three or four parts of

water; but this has not been recommended amongst the “preservative

liquids,” as the colour, which is often an attractive quality of this

class of objects, is thereby destroyed.

We have now noticed the treatment which must be applied to those

objects which are to be preserved in liquids and cells. We may here

state that all slides of this kind should be examined at short

intervals, as they will be found now and then to require another

coating of varnish round the edge of the thin glass cover to prevent

all danger of leakage. The use of the air-pump, in the first instance

(as before recommended), and this precaution as to the varnish, will

render the slides less liable to leakage and air-bubbles, which so very

frequently render them almost worthless.

CHAPTER V.

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 special 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 upon 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 “laminæ,” which

require nothing but mounting in Canada balsam to show the texture very

well. In working, however, 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, 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 getting 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,

Diatomaceæ, &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 dried 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. The

“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 “_lacunæ_,” 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’s 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,

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 matter 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 uniform 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

orchideous 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 thoroughly and oil them before laying by, if the place is at all

damp. This instrument is most useful in such subjects as anatomical

preparations where the sections are required to show the position of

the different vessels, &c.; but, as before stated, is very valuable for

all soft substances. As an instance of this, it may be mentioned, that

it is frequently used in cutting sections of sponges; but as these are

often very full of spicula, &c., it is much better to press the sponge

flat until dry, and then cut off thin shavings with a very sharp knife;

these shavings will expand when placed in water. After this they may

be laid betwixt two flat surfaces and dried, when they may be mounted

as other dry objects, or, when desirable, in balsam.

_Valentin’s knife_ is very much used in taking sections of skin, which

are afterwards treated with potash solution, acids, &c., to bring out

in the best way the different portions. Dr. Lister’s mode, however,

of getting these is thus given in the _Microscopic Journal_:--“But

I afterwards found that much better sections could be obtained from

dried specimens. A portion of shaved scalp being placed between two

thin slips of deal, a piece of string is tied round them so as to

exercise a slight degree of compression; the preparation is now laid

aside for twenty-four hours, when it is found to be dried to an almost

horny condition. It then adheres firmly by its lower surface to one

of the slips, and thus it can be held securely, while extremely thin

and equable sections are cut with great facility in any plane that may

be desired. These sections, when moistened with a drop of water and

treated with acetic acid, are as well suited for the investigation of

the muscular tissue as if they had not been dried.”

There are many who almost confine their attention to polarized light

and its beautiful effects. Such would not deem these efforts to aid

the student in cutting sections complete without some notice of those

which are taken from various crystals, in order to display that curious

and beautiful phenomenon, _the rings with a cross_. The arrangement of

these is somewhat changed by the crystal which affords the section; but

nitrate of potash gives two sets of rings with a cross, the long line

of which passes through both, the short line dividing it in the middle.

The process of cutting these sections is rather difficult, but a

little care and perseverance will conquer all this. The following is

extracted from the _Encyclopædia Metropolitana_:--“Nitre crystallizes

in long six-sided prisms whose section, perpendicular to their sides,

is the regular hexagon. They are generally very much interrupted in

their structure; but by turning over a considerable quantity of the

ordinary saltpetre[F] of the shops specimens are readily found which

have perfectly transparent portions of some extent. Selecting one

of these, cut it with a knife into a plate _above_ a quarter of an

inch thick, directly across the axis of the prism, and then grind it

down on a broad wet file till it is reduced to about one quarter or a

sixth of an inch thick, smooth the surface on a wet piece of emeried

glass, and polish on a piece of silk strained very tight over a strip

of plate-glass, and rubbed with a mixture of tallow and colcothar

of vitriol. This operation requires practice. It cannot be effected

unless the nitre be applied wet and rubbed till quite dry, increasing

the rapidity of the friction as the moisture evaporates. It must be

performed in gloves, as the vapour from the fingers, as well as the

slightest breath, dims the polished surface effectually. With these

precautions a perfect vitreous polish is easily obtained. We may here

remark, that hardly any two salts can be polished by the same process.

Thus, Rochelle-salt must be finished wet on the silk, and instantly

transferred to soft bibulous linen and rapidly rubbed dry. Experience

alone can teach these peculiarities, and it is necessary to resort to

contrivances (sometimes very strange ones) for the purpose of obtaining

good polished sections of soft crystals, especially of those easily

soluble in water.

“The nitre is thus polished on both its surfaces, which should be

brought as near as possible to parallelism.”

[F] Sometimes the saltpetre of the shops is nitrate of _soda_,

and as this is slightly deliquescent, it is well to be

certain that we have the nitrate of _potash_, which is free

from this defect.

Some sections of the naturally formed crystals also show the “rings”

very well,--as Iceland Spar, which gives a single ring and cross; but

the difficulty of cutting and polishing them is almost too great for

the amateur, and must be left to the lapidary. This curious phenomenon,

however, may be seen by using a plate of ice uninterruptedly formed of

about one inch in thickness.

Before concluding these remarks on sections, I must mention a few

difficulties which may be met with, and their remedies. The foremost of

these is the softness of some objects, which have not resistance enough

in themselves to bear cutting even with the sharpest instruments.

This may often be removed by soaking in a solution of gum, and then

drying, which will render the substance firm enough to be cut, when the

sections must be steeped in water, and the gum thus got rid of. Small

seeds, &c., may be placed in wax when warmed, and will be held firmly

enough when it is again cold to allow of them being cut into sections,

&c. And, lastly, where a substitute for a microscopist’s hand-vice is

required, a cork which fits any tube large enough may be taken and

split, the object being then placed between the two parts, and the cork

thrust into the tube, a sufficient degree of firmness will be obtained

to resist any necessary cutting, &c.

DISSECTION.--As I stated at the commencement of this chapter, no

written instructions can enable any student to become an adept in this

branch without much experience and no little study. I will, however,

describe the necessary apparatus, and afterwards mention the mode of

treatment which certain objects require.

A different microscope is manufactured for the purpose of dissection,

most first-rate makers having their own model. The object-glasses of

many of these are simple, and consequently not expensive; but one of

the great requisites is a stage large enough to hold the trough, in

which the operation is often performed. Where this is the case it would

scarcely be worth the expense of getting a dissecting microscope if the

student were pursuing no particular study, but merely made use of the

instrument when an object to be operated upon turned up accidentally.

The ordinary form is much improved for this purpose, by having two

wooden rests placed at the sides of the microscope, upon which the

hands may be supported when working upon the stage. These should be

weighty enough to be free from danger of moving. These supports will

also be found to be a remedy against much of the weariness which

inevitably arises from having to sustain the hands as well as work

with them. The erector, as I before observed, is necessary to a young

student; but with a little practice he may work very well without it.

We will now notice some of the instruments which are most useful in

dissection. Two or three different sizes of ordinary scissors should

be possessed, but the shapes must be modified in others for many

purposes, as those used by surgeons; a pair with the cutting parts

bent in a horizontal direction, and another pair slightly curved in

a perpendicular; so that parts of the substance operated upon may be

reached, which it would be impossible to touch with straight scissors.

One point of these is sometimes blunt, and the other acute, being

thus made very useful in opening tubular formations. Another form of

these is made, where the blades of the scissors are kept open by a

spring, the handles being pressed together by the fingers. Where it is

desirable, one or both of these handles may be lengthened to any degree

by the addition of small pieces of wood.

THE KNIVES which are most useful are those of the smallest kind which

surgeons employ in very delicate operations. These are made about the

length of an ordinary pen-knife, and are fixed in rather long flattish

handles; some are curved inwards, like the blade of a scythe, others

backwards; some taper to a point, whilst others again are broad and

very much rounded. Complete boxes are now fitted up by the cutlers, of

excellent quality and surprisingly cheap.