CHAPTER VII.

MISCELLANEOUS. 140

INDEX. 153

THE

PREPARATION AND MOUNTING

OF

MICROSCOPIC OBJECTS.

CHAPTER I.

APPARATUS.

Before entering into the subject of the setting of Objects for

the Microscope, the student must be convinced of the necessity of

cleanliness in everything relating to the use of that instrument. In

no branch is this more apparent than in the _preparation_ of objects;

because a slide which would be considered perfectly clean when viewed

in the ordinary way is seen to be far otherwise when magnified some

hundreds of diameters; and those constant enemies, the floating

particles of dust, are everywhere present, and it is only by unpleasant

experience that we fully learn what _cleanliness is_.

Any object which is to be viewed under the microscope must, of course,

be supported in some way--this is now usually done by placing it upon a

glass slide, which on account of the transparency has a great advantage

over other substances. These “slides” are almost always made of one

size, viz., three inches long by one broad, generally having the edges

ground so as to remove all danger of scratching or cutting any object

with which they may come in contact. The glass must be very good,

else the surface will always present the appearance of uncleanliness

and dust. This dusty look is very common amongst the cheaper kinds of

slides, because they are usually “sheet” glass; but is seldom found

in those of the quality known amongst dealers by the name of “patent

plate.” This latter is more expensive at first, but in the end there

is little difference in the cost, as so many of the cheaper slides

cannot be used for delicate work if the mounted object is to be seen in

perfection. These slides vary considerably in thickness; care should,

therefore, be taken to sort them, so that the more delicate objects

with which the higher powers are to be used maybe mounted upon the

_thinnest_, as the light employed in the illumination is then less

interfered with. To aid the microscopist in this work, a metal circle

may be procured, having a number of different sized openings on the

outer edge, by which the glass slides can be measured. These openings

are numbered, and the slides may be separated according to these

numbers; so that when mounting any object there will be no need of a

long search for that glass which is best suited to it.

When fresh from the dealer’s hands, these slides are generally

covered with dust, &c., which may be removed by well washing in clean

rain-water; but if the impurity is obstinate, a little washing soda

may be added, care being taken, however, that every trace of this is

removed by subsequent waters, otherwise the crystals will afterwards

form upon the surface. A clean linen cloth should be used to dry the

slides, after which they may be laid by for use. Immediately, however,

before being used for the reception of objects by any of the following

processes, all dust must be removed by rubbing the surface with clean

wash-leather or a piece of cambric, and, _if needful_, breathing upon

it, and then using the leather or cambric until perfectly dry. Any

small particles left upon the surface may generally be removed by

blowing gently upon it, taking care to allow no damp to remain.

We have before said, that any object to be viewed in the microscope

must have its support; but if this object is to be preserved, care

must be taken that it is defended from the dust and other impurities.

For this purpose it is necessary to use some transparent cover, the

most usual at one time being a plate of mica, on account of its

thinness; this substance is now, however, never used, thin glass being

substituted, which answers admirably. Sometimes it is required to

“_take up_” as little space as possible, owing to the shortness of

focus of the object-glasses. It can be procured of any thickness, from

one-fiftieth to one-two-hundred-and-fiftieth of an inch. On account

of its want of strength it is difficult to cut, as it is very liable

to “_fly_” from the point of the diamond. To overcome this tendency

as much as possible, it must be laid upon a thicker piece, previously

made wet with water, which causes the thin glass to adhere more firmly,

and consequently to bear the pressure required in cutting the covers.

The process of cutting being so difficult, especially with the thinner

kinds, little or nothing is gained by cutting those which can be got

from the dealers, as the loss and breakage is necessarily greater in

the hands of an amateur. It is convenient, however, to have on hand a

few larger pieces, from which unusual sizes may be cut when required.

If the pieces required are _rectangular_, no other apparatus will be

required save a diamond and a flat rule; but if _circles_ are wanted,

a machine for that purpose should be used (of which no description

is necessary here). There are, however, other contrivances which

answer tolerably well. One method is, to cut out from a thick piece of

cardboard a circle rather larger than the size wanted. Dr. Carpenter

recommends metal rings with a piece of wire soldered on either side;

and this, perhaps, is the best, as cardboard is apt to become rough at

the edge when much used. A friend of mine uses thin brass plates with

circles of various sizes “turned” through them, and a small raised

handle placed at one end. The diamond must be passed round the inner

edge, and so managed as to meet again in the same line, in order that

the circle may be true, after which they may be readily disengaged. The

sizes usually kept in stock by the dealers are one-half, five-eighths,

and three-quarters inch diameter; but other sizes may be had to order.

For the information of the beginner it may be mentioned here that the

price of the circles is a little more than of the squares; but this

is modified in some degree by the circle being rather lighter. If

appearance, however, is cared for at all, the circles look much neater

upon the slides when not covered with the ornamental papers; but if

these last are used (as will shortly be described) the squares are

equally serviceable.

As before mentioned, the thin glass is made of various thicknesses,

and the beginner will wish to know which to use. For objects requiring

no higher power than the one-inch object-glass, the thicker kinds

serve well enough; for the half-inch the medium thickness will be

required; while, for higher powers, the thinnest covers must be used.

The “test-objects” for the highest powers require to be brought so

near to the object-glass that they admit of the very thinnest covering

only, and are usually mounted betwixt glasses which a beginner would

not be able to use without frequent breakage; but if these objects were

mounted with the common covers, they would be really worthless with the

powers which they require to show them satisfactorily.

It may be desirable to know how such small differences as those betwixt

the various thin glass covers can be measured. For this purpose there

are two or three sorts of apparatus, all, however, depending upon the

same principle. The description of one, therefore, will be sufficient.

Upon a small stand is a short metal _lever_ (as it may be termed) which

returns by a spring to one certain position, where it is in contact

with a fixed piece of metal. At the other end this lever is connected

with a “finger,” which moves round a dial like that of a watch,

whereupon are figures at fixed distances. When the lever is separated

from the metal which is stationary, the other end being connected

with the “finger” of the dial, that “finger” is moved in proportion

to the distance of the separation. The thin glass is, therefore,

thrust betwixt the end of the lever and fixed metal, and each piece is

measured by the figures on the dial in stated and accurate degrees.

This kind of apparatus, however, is expensive, and when not at our

command, the thin glass may be placed edgewise in the stage forceps,

and measured very accurately with the micrometer.

Cleanliness with the thin glass is, perhaps, more necessary than with

the slides, especially when covering objects which are to be used with

a high power; but it is far more difficult to attain, on account of

the liability to breakage. The usual method of cleaning these covers

is as follows:--Two discs of wood, about two inches in diameter, are

procured, one side of each being perfectly flat and covered with clean

wash-leather. To the other side of these a small knob is firmly affixed

as a handle, or where practicable the whole may be made out of a solid

piece. In cleaning thin glass, it should be placed betwixt the covered

sides of the discs, and may then be safely rubbed with a sufficient

pressure, and so cleaned on both sides by the leather. If, however,

the glass be greasy, as is sometimes the case, it must be first washed

with a strong solution of potash, infusion of nut-galls, or any of

the commonly used grease-removing liquids; and with _some_ impurities

water, with the addition of a few drops of strong acid, will be found

very useful, but this last is not often required.

The above method of cleaning thin glass should always be used by

beginners; but after some experience the hand becomes so sensitive

that the above apparatus is often dispensed with, and the glasses,

however thin, may be safely cleaned betwixt the fingers and thumb with

a cambric handkerchief, having first slightly damped the ends of the

fingers employed to obtain firm hold. When the dirt is very obstinate,

breathing upon the glass greatly facilitates its removal, and the sense

of touch becomes so delicate that the breakage is inconsiderable; but

this method cannot be recommended to novices, as nothing but time spent

in delicate manipulation can give the sensitiveness required.

It was before mentioned that the ordinary glass slides were sometimes

worthless, _especially for fine objects_, from having a rough surface,

which presented a dusty appearance under the microscope. This

imperfection exists in some _thin glass_ also, and it is irremediable;

so that it is useless to attempt to cleanse it; nevertheless, care

should be taken not to mistake dirty glass for this roughness, lest

good glass be laid aside for a fault which does not really belong to it.

When any object which it is desired to mount is of considerable

thickness, or will not bear pressure, it is evident that a wall must

be raised around it to support the thin glass--this is usually termed

a “cell.” There are various descriptions of these, according to the

class of objects they are required to protect; and here may be given a

description of those which are most generally used in mounting “dry”

objects, leaving those required for the preservation of liquids until

we come to the consideration of that mode of mounting. Many have made

use of the following slides. Two pieces of hard wood of the usual size

(3 in. by 1 in.), not exceeding one-sixteenth of an inch in thickness,

are taken, and a hole is then drilled in the middle of one of these

of the size required. The two pieces are then united by glue or other

cement, and left under pressure until thoroughly dry, when the cell is

fit for use. Others substitute cardboard for the lower piece of wood,

which is less tedious, and is strong enough for every purpose. This

class of “cell” is, of course, fitted for opaque objects only where

no light is required from below; and as almost all such are better

seen when on a dark background, it is usual to fix a small piece of

black paper at the bottom of the cell upon which to place them. For

very small objects the “grain” which all such paper has when magnified

detracts a little from the merit of this background; and lately I

have used a small piece of thin glass covered on the back with black

varnish, and placed the object upon the smooth untouched side.

Another method of making these cells is as follows:--Two “punches,”

similar to those used for cutting “gun-wads,” are procured, of such

sizes that with the smaller may be cut out the centre of the larger,

leaving a ring whose side is not less than one-eighth of an inch wide.

These rings may be readily made, the only difficulty being to keep the

sides parallel; but a little care will make this easy enough. For this

purpose close-grained cardboard may be conveniently used. It must have

a well-glazed surface, else the varnish or cement used in affixing the

thin glass cover sinks into the substance, and the adherence is very

imperfect. When this takes place it is easily remedied by brushing over

the surface of the cardboard a strong solution of gum or isinglass; and

this application, perhaps, closes also the pores of the card, and so

serves a double purpose. But, of course, the gum must be _perfectly_

dried before the ring is used.

For cardboard, gutta-percha has been substituted, but cannot be

recommended, as it always becomes brittle after a certain time, never

adheres to the glass with the required firmness, and its shape is

altered when worked with even a little heat. Leather is often used, and

is very convenient; it should be chosen, however, of a close texture,

and free from oil, grease, and all those substances which are laid upon

it by the “dressers.”

Rings of cardboard, &c., have been rejected by persons of great

experience, because they are of such a nature that dampness can

penetrate them. This fault can be almost, if not totally, removed by

immersing them in some strong varnish, such as the asphalt varnish

hereinafter mentioned; but they must be left long enough when affixed

to the glass slide to become _perfectly dry_, and this will require a

much longer time than at first would be supposed.

There has, however, been lately brought out what is termed the _ivory

cell_. This is a ring of ivory-like substance, which may be easily and

firmly fixed to the glass slide by any of the commonly-used cements,

and so forms a beautiful cell for any dry objects. They are made of

different sizes, and are not expensive.

Sometimes slides are used which are made by taking a thin slip of wood

of the usual size (3 in. by 1 in.), in the centre of which is cut a

circular hole large enough to receive the object. A piece of thin

glass is fixed underneath the slide, forming a cell for the object,

which may then be covered and finished like an ordinary slide. This

has the advantage of serving for transparent objects for which the

before-mentioned wooden slides are unsuitable. A slight modification

of this plan is often used where the thickness of the objects is

inconsiderable, especially with some of the Diatomaceæ, often termed

“test-objects.” The wooden slide is cut with the central opening as

above, and two pieces of thin glass are laid upon it, betwixt which the

diatoms or other objects are placed, and kept in their proper position

by a paper cover. This arrangement is a good one, insomuch as the very

small portion of glass through which the light passes on its way to the

microscope from the reflector causes the refraction or interference to

be reduced to the lowest point.

A novice would naturally think the appearance of some of the slides

above mentioned very slovenly and unfinished; but they are often

covered with ornamental papers, which may be procured at almost every

optician’s, at a cost little more than nominal, and of innumerable

patterns and colours. How to use these will be described in another

place.

It is very probable that a beginner would ask his friend what kind of

slides he would advise him to use. Almost all those made of wood are

liable to warp more or less, even when the two pieces are separate or

of different kinds; those of cardboard and wood are generally free

from this fault, yet the slides, being opaque, prevent the employment

of the Lieberkuhn. To some extent glass slides, when covered with

ornamental papers, are liable to the same objection, as the light is

partly hindered. And sometimes the dampness from the paste, or other

substance used to affix the papers, penetrates to the object, and so

spoils it, though this may be rendered less frequent by first attaching

the _thin_ glass to the slide by some harder cement. Much time,

however, is taken up by the labour of covering the slides, which is a

matter of consideration with some. Certainly, the cost of the glass

slides was formerly great; but now they are reasonable enough in this

respect, so that this objection is removed. It is, therefore, well to

use glass slides, except where the thin glasses are employed for tests,

&c., as above. When the thin glass circles are placed upon the slides,

and the edge is varnished with black or coloured rings, the appearance

of finish is perfect. The trouble is much less than with most of

the other methods, and the illumination of the object very slightly

interfered with.

To varnish the edges of these covers, make circles of any liquid upon

the glass slide, and perform any other “circular” work mentioned

hereafter, the little instrument known as “Shadbolt’s turntable” is

almost indispensable. It is made as follows:--At one end of a small

piece of hard wood is fixed an iron pivot about one-eighth inch thick,

projecting half an inch from the wood, which serves as a centre upon

which a round brass table three inches in diameter revolves. On the

surface of this are two springs, about one and a half inches apart,

under which the slide is forced and so kept in position, whilst the

central part is left open to be worked upon. The centre is marked, and

two circles half an inch and one inch in diameter are usually deeply

engraved upon the table to serve as guides in placing the slide, that

the ring may be drawn in the right position. When the slide is placed

upon the “table” underneath the springs, a camel-hair pencil is filled

with the varnish, or other medium used, and applied to the surface

of the glass; the table is then made to revolve, and a circle is

consequently produced, the diameter of which it is easy to regulate.

Many objects for the microscope may be seriously injured by allowing

the fingers to touch them--many more are so minute that they cannot

be removed in this way at all, and often it is necessary to take

from a mass of small grains, as in sand, some particular particle.

To accomplish this, there are two or three contrivances recommended:

one by means of split bristles, many of which will readily be found

in any shaving brush when it has been well used. The bristles when

pressed upon any hard surface open, and when the pressure is removed

close again with a spring; but the use of these is limited. Camel-hair

pencils are of great service for this and many other purposes to the

microscopist. In _very_ fine work they are sometimes required so small

that all the hairs with the exception of one or two finer pointed ones

are removed. A few of various sizes should always be kept on hand.

Equally necessary are fine pointed needles. They are very readily put

up for use by thrusting the “eye” end into a common penholder, so as to

be firm. The points may be readily renewed, when injured, on a common

whetstone; but when out of use they may be protected by being thrust

into a piece of cork.

Knives of various kinds are required in some branches of microscopic

work, but these will be described where “dissection,” &c., is treated

at some length, as also various forms of scissors. In the most simple

objects, however, scissors of the usual kind are necessary. Two or

three sizes should always be kept at hand, sharp and in good order.

A set of glass tubes, kept in a case of some sort to prevent breakage,

should form part of our “fittings” and be always cleaned immediately

after use. These are generally from six to ten inches long and from

one-eighth to a quarter of an inch in diameter. One of these should

be straight and equal in width at both ends; one should be drawn out

gradually to a fine point; another should be pointed as the last, but

be slightly curved at the compressed end, in order to reach points

otherwise unattainable. It is well to have these tubes of various

widths at the points, as in some waters the finer would be inevitably

stopped. For other purposes the fine ones are very useful, especially

in the transfer of “preservative liquids” which will come under notice

in another chapter.

Forceps are required in almost all microscopic manipulations, and

consequently are scarcely ever omitted from the microscopic box,

even the most meagrely furnished; but of these there are various

modifications, which for certain purposes are more convenient than the

usual form. The ordinary metal ones are employed for taking up small

objects, thin glass, &c.; but when slides are to be held over a lamp,

or in any position where the fingers cannot conveniently be used, a

different instrument must be found. Of these there are many kinds; but

Mr. Page’s wooden forceps serve the purpose very well. Two pieces of

elastic wood are strongly bound together at one end, so that they may

be easily opened at the other, closing again by their own elasticity.

Through the first of these pieces is loosely passed a brass stud,

resembling a small screw, and fastened in the second, and through the

second a similar stud is taken and fixed in the first--so that on

pressure of the studs the two strips of wood are opened to admit a

slide or other object required to be held in position. The wood strips

are generally used three or four inches long, one inch wide, and about

one-eighth inch thick.

Again, some objects when placed upon the glass slide are of such an

elastic nature that no cement will secure the thin glass covering until

it becomes hard. This difficulty may be overcome by various methods.

The following are as good and simple as any. Take two pieces of wood

about two inches long, three-quarters wide, and one-quarter thick;

and a small rounded piece one inch long and one-quarter in diameter;

place this latter betwixt the two larger pieces. Over one end of the

two combined pass an india-rubber band. This will give a continual

pressure, and may be opened by bringing the two pieces together at

the other end; the pressure may be readily made uniform by paring the

points at the inner sides, and may be regulated by the strength of the

india-rubber band. These bands may be made cheaply, and of any power,

by procuring a piece of india-rubber tubing of the width required, and

cutting off certain breadths. Another very simple method of getting

this pressure is mentioned in the “Micrographic Dictionary.” Two pieces

of whalebone of the length required are tied together firmly at each

end. It is evident that any object placed betwixt them will be subject

to continual pressure. The power of this may be regulated by the

thickness and length of the whalebone. This simple contrivance is very

useful.

Common watch-glasses should always be kept at hand. They are certainly

the cheapest, and their transparency makes them very convenient

reservoirs in which objects may be steeped in any liquid; as it

saves much trouble to examine cursorily under the microscope, when

the air-bubbles are expelled from insects, &c., &c. They are readily

cleaned, and serve very well as covers, when turned upside down,

to protect any objects from the dust. For this latter purpose Dr.

Carpenter recommends the use of a number of bell-glasses, especially

when one object must be left for a time (which often happens) in order

that another may be proceeded with. Wine glasses, when the “legs” are

broken, may thus be rendered very useful.

As heat is necessary in mounting many objects, a lamp will be required.

Where gas is used, the small lamp known as “Bunsen’s” is the most

convenient and inexpensive. It gives great heat, is free from smoke,

and is readily affixed to the common gas-burner by a few feet of

india-rubber tubing. The _light_ from these lamps is small, but this is

little or no drawback to their use. Where gas is not available, the

common spirit-lamps may be used, which are very clean and answer every

purpose.

In applying the required heat to the slides, covers, &c., it is

necessary in many cases to ensure uniformity, otherwise there is danger

of the glass being broken. For this purpose a brass plate at least

three inches wide, somewhat longer, and one-eighth of an inch thick,

must be procured. It should then be affixed to a stand, so that it may

be readily moved higher or lower, in order that the distance from the

lamp may be changed at will, and thus the degree of heat more easily

regulated. This has also the advantage of enabling the operator to

allow his slides, &c., to cool more gradually, which, in some cases, is

absolutely necessary,--as in fusing some of the salts, &c.

In order to get rid of air-bubbles, which are frequently disagreeable

enemies to the mounter of objects, an air-pump is often very useful.

This is made by covering a circular plate of metal with a bell-glass,

both of which are ground so finely at the edges that greasing the place

of contact renders it air-tight. The pump is then joined to the metal

plate underneath, and worked with a small handle like a common syringe.

By turning a small milled head the air may be allowed to re-enter when

it is required to remove the bell-glass and examine or perform any

operation upon the object. The mode of using this instrument will be

described hereafter, but it may be here stated that substitutes have

been devised for this useful apparatus; but as it is now to be obtained

at a low cost, it is hardly worth while to consider them. Much time is,

in many instances, certainly saved by its use, as a very long immersion

in the liquids would be required to expel the bubbles, where the

air-pump would remove them in an hour.

The next thing to be considered is what may be termed CEMENTS, some

of which are necessary in every method of mounting objects for the

microscope. Of these will be given the composition where it is

probable the young student can make use of it; but many of them are

so universally kept as to be obtainable almost anywhere; and when

small quantities only are required, economy suffers more from home

manufacture than from paying the maker’s profit.

Amongst these, CANADA BALSAM may, perhaps, be termed the most

necessary, as it is generally used for the preservation of many

transparent objects. It is a thick liquid resin of a light amber

colour, which on exposure to the atmosphere becomes dry and hard

even to brittleness. For this reason it is seldom used as a _cement_

alone where the surface of contact is small, as it would be apt to be

displaced by any sudden shock, especially when old. In the ordinary

method of using, however, it serves the double purpose of preserving

the object and fixing the thin glass cover; whilst the comparatively

large space upon which it lies lessens the risk of displacement. By

keeping, this substance becomes thicker; but a very little warmth will

render it liquid enough to use even when to some extent this change has

taken place. When heated, however, for some time and allowed to cool,

it becomes hardened to any degree, which may be readily regulated by

the length of time it has been exposed, and the amount of heat to which

it has been subjected. On account of this property it is often used

with chloroform: the balsam is exposed to heat until, on cooling, it

assumes a glassy appearance; it is then dissolved in pure chloroform

until it becomes of the consistence of thick varnish. This liquid is

very convenient in some cases; as air-bubbles are much more easily

got rid of than when undiluted Canada balsam is used. It also dries

readily, as the chloroform evaporates very quickly, for which reason it

must be preserved in a closely-stoppered bottle. It has been said that

this mixture becomes _cloudy_ with long keeping, but I have not found

it so in any cases where I have used it. Should it, however, become so,

a little heat will readily dispel the opacity. The ordinary balsam,

if exposed much to the air whilst being used, becomes thicker, as has

been already stated. It may be reduced to the required consistency

with common turpentine, but I have often found this in some degree

injurious to the transparency of the balsam, and the amalgamation of

the two is by no means perfect. (See also Chapter III.) The cheapness

of the article renders it no extravagance to use it always undiluted;

and when preserved in a bottle with a hollow cover fitting tightly

around the neck, both surfaces being finely ground, it remains fit for

use much longer than in the ordinary jar.

ASPHALTUM.--This substance is dissolved in linseed oil, turpentine, or

naphtha, and is often termed “Brunswick black.” It is easily worked,

but is not generally deemed a _trustworthy_ cement, as after a time it

is readily loosened from its ground. It is, however, very useful for

some purposes (such as “finishing” the slides), as it dries quickly. I

shall, however, mention a modification of this cement a little further

on.

MARINE GLUE.--No cement is more useful or trustworthy for certain

purposes than this. It is made in various proportions; but one really

good mixture is--equal parts of india-rubber and gum shellac; these are

dissolved in mineral naphtha with heat. It is, however, much better to

get it from the opticians or others who keep it. It requires heat in

the application, as will be explained in Chapter IV.; but it is soluble

in few, if any, liquids used by the microscopist, and for that reason

is serviceable in the manufacture of cells, &c. Where two pieces of

glass are to be firmly cemented together, it is almost always employed;

and in all glass troughs, plates with ledges, &c., the beginner may

find examples of its use.

GOLD SIZE.--This substance may always be procured at any colourman’s

shop. The process of its preparation is long and tedious. It is,

therefore, not necessary to describe it here. Dr. Carpenter says that

it is very durable, and may be used with almost any preservative

liquids, as it is acted upon by very few of them, turpentine being its

only true solvent. If too thin, it may be exposed for awhile to the

open air, which by evaporation gradually thickens it. Care must be

taken, however, not to render it too thick, as it will then be useless.

A small quantity should be kept on hand, as it is much more adhesive

when _old_.

LIQUID GLUE is another of these cements, which is made by dissolving

gum shellac in naphtha in such quantity that it may be of the required

consistency. This cement appears to me almost worthless in ordinary

work, as its adherence can never be relied upon; but it is so often

used and recommended that an enumeration of cements might be deemed

incomplete without it. Even when employed simply for varnishing the

outside of the glass covers, for appearance’ sake alone, it invariably

chips. Where, however, oil is used as a preservative liquid, it serves

very well to attach the thin glass; but when this is accomplished,

another varnish less liable to “chip” must always be laid upon it. (See

Chapter IV.)

BLACK JAPAN.--This is prepared from oil of turpentine, linseed oil,

amber, gum anime, and asphalt. It is troublesome to make, and therefore

it is much better to procure it at the shops. It is a really good

cement, and serves very well to make shallow cells for liquids, as will

be described in Chapter IV. The finished cell should be exposed for a

short time to the heat of what is usually termed a “cool oven.” This

renders it very durable, and many very careful manipulators make use of

it for their preparations.

ELECTRICAL CEMENT.--This will be found very good for some purposes

hereinafter described. To make it, melt together--

5 parts of resin.

1 ” beeswax.

1 ” red ochre.[A]

[A] Dr. Griffiths says that the addition of 2 parts of Canada

balsam renders this cement much more adhesive to the glass.

It must be used whilst hot, and as long as it retains even slight

warmth can be readily moulded into any form. It is often employed in

making shallow cells for liquids, as before mentioned.

GUM-WATER is an article which nobody should ever be without; but

labels, or indeed any substance, affixed to glass with common gum, are

so liable to leave it spontaneously, especially when kept very dry,

that I have lately added five or six drops of glycerine to an ounce of

the gum solution. This addition has rendered it very trustworthy even

on glass, and now I never use it without. This solution cannot be kept

long without undergoing fermentation, to prevent which the addition

of a small quantity of any essential oil (as oil of cloves, &c.),

or one-fourth of its volume of alcohol, may be made, which will not

interfere in any way with its use.

There is what is sometimes termed an _extra adhesive_ gum-water, which

is made with the addition of isinglass, thus:--Dissolve two drachms of

isinglass in four ounces of distilled vinegar; add as much gum arabic

as will give it the required consistency. This will keep very well, but

is apt to become thinner, when a little more gum may be added.

All these, except one or two, are liquid, and must be kept in

_stoppered_ bottles, or, at least, as free from the action of the air

as possible.

When any two substances are to be united firmly, I have termed the

medium employed “a cement;” but often the appearance of the slides is

thought to be improved by drawing a coloured ring upon them, extending

partly on the cover and partly on the slide, hiding the junction of the

two. The medium used in these cases I term A VARNISH, and hereinafter

mention one or two. Of course, the tenacity is not required to be so

perfect as in the _cements_.

SEALING-WAX VARNISH is prepared by coarsely powdering sealing wax,

and adding spirits of wine; it is then digested at a gentle heat to

the required thickness. This is very frequently used to “finish” the

slides, as before mentioned, and can easily be made of any colour by

employing different kinds of sealing-wax; but is very liable to “chip”

and leave the glass.

BLACK VARNISH is readily prepared by adding a small quantity of

lamp-black to gold-size and mixing intimately. Dr. Carpenter recommends

this as a good finishing varnish, drying quickly and being free from

that brittleness which renders some of the others almost worthless; but

it should not be used in the first process when mounting objects in

fluid.

Amongst these different cements and varnishes I worked a long time

without coming to any decision as to their comparative qualities,

though making innumerable experiments. The harder kinds were

continually cracking, and the softer possessed but little adhesive

power. To find hardness and adhesiveness united was my object, and the

following possesses these qualities in a great degree:--

India-rubber ½ drachm.

Asphaltum 4 oz.

Mineral naphtha 10 ”

Dissolve the india-rubber in the naphtha, then add the asphaltum--if

necessary heat must be employed.

This is often used by photographers as a black varnish for glass, and

never cracks, whilst it is very adhesive. Dr. Carpenter, however,

states that his experience has not been favourable to it; but I have

used it in great quantities and have never found it to leave the glass

in a single instance when used in the above proportions. The objections

to it are, however, I think easily explained, when it is known that

there are many kinds of pitch, &c., from coal, sold by the name of

asphaltum, some of which are worthless in making a microscopic cement.

When used for this purpose, the asphaltum must be genuine and of the

best quality that can be bought. The above mixture serves a double

purpose--to unite the cell to the slide, and also as a “finishing”

varnish. But it is perhaps more convenient to have two bottles of this

cement, one of which is thicker than common varnish, to use for uniting

the cell, &c.; the other liquid enough to flow readily, which may be

employed as a surface varnish in finishing the slides.

The brushes or camel-hair pencils should always be cleaned after use;

but with the asphalt varnish above mentioned it is sufficient to wipe

off as carefully as possible the superfluous quantity which adheres to

the pencil, as, when again made use of, the varnish will readily soften

it; but, of course, it will be necessary to keep separate brushes for

certain purposes.

Here it may be observed that every object should be labelled with name

and any other descriptive item as soon as mounted. There are many

little differences in the methods of doing this. Some write with a

diamond upon the slide itself; but this has the disadvantage of being

not so easily seen. For this reason a small piece of paper is usually

affixed to one end of the slide, on which is written what is required.

These labels may be bought of different colours and designs; but the

most simple are quite as good, and very readily procured. Take a sheet

of thin writing paper and brush over one side a strong solution of gum,

with the addition of a few drops of glycerine as above recommended;

allow this to dry, and then with a common gun-punch stamp out the

circles, which may be affixed to the slides by simply damping the

gummed surface, taking care to write the required name, &c., upon it

before damping it, or else allowing it to become perfectly dry first.

There is one difficulty which a beginner often experiences in

sorting and mounting certain specimens under the microscope, viz.,

the _inversion_ of the objects; and it is often stated to be almost

impossible to work without an erector. But this difficulty soon

vanishes, the young student becoming used to working what at first

seems in contradiction to his sight.

Let it be understood, that in giving the description of those articles

which are usually esteemed _necessary_ in the various parts of

microscopic manipulation, I do not mean to say that without many of

these no work of any value can be done. There are, as all will allow,

certain forms of apparatus which aid the operator considerably; but

the cost may be too great for him. A little thought, however, will

frequently overcome this difficulty, by enabling him to make, or

get made, for himself, at a comparatively light expense, something

which will accomplish all he desires. As an example of this, a friend

of mine made what he terms his “universal stand,” to carry various

condensers, &c., &c., in the following way:--Take a steel or brass

wire, three-sixteenths or one-quarter inch thick and six or eight

inches long; “tap” into a _solid_, or make rough and fasten with melted

lead into a _hollow_, ball. (The foot of a cabinet or work-box answers

the purpose very well.) In the centre of a round piece of tough board,

three inches in diameter, make a hemispherical cavity to fit half of

the ball, and bore a hole through from the middle of this cavity, to

allow the wire to pass. Take another piece of board, about four inches

in diameter, either round or square, and one-and-a-half or two inches

thick, make a similar cavity in its centre to receive the other half

of the ball, but only so deep as to allow the ball to fit tightly when

the two pieces of board are screwed together, which last operation

must be done with three or four screws. Let the hole for the wire in

the upper part be made conical (base upwards), and so large as only to

prevent the ball from escaping from its socket, in order that the shaft

may move about as freely as possible. Turn a cavity, or make holes,

in the bottom of the under piece, and fill with lead to give weight

and steadiness. This, painted green bronze and varnished, looks neat;

and by having pieces of gutta-percha tubing to fit the shaft, a great

variety of apparatus may be attached to it.

Again, a “condenser” is often required for the illumination of opaque

objects. My ingenious friend uses an “engraver’s bottle” (price 6d.),

fills it with water, and suspends it betwixt the light and the object.

Where the light is very yellow, he tints the water with indigo, and so

removes the objectionable colour.

I merely mention these as examples of what may be done by a little

thoughtful contrivance, and to remove the idea that nothing is of

much value save that which is the work of professional workmen, and

consequently expensive.

CHAPTER II.

TO PREPARE AND MOUNT OBJECTS “DRY.”

The term “dry” is used when the object to be mounted is not immersed

in any liquid or medium, but preserved in its natural state, unless it

requires cleaning and drying.

I have before stated that thorough cleanliness is necessary in the

mounting of all microscopic objects. I may here add that almost every

kind of substance used by the microscopist suffers from careless

handling. Many leaves with fine hairs are robbed of half their beauty,

or the hairs, perhaps, forced into totally different shapes and groups;

many insects lose their scales, which constitute their chief value to

the microscopist; even the glass itself distinctly shows the marks of

the fingers if left uncleaned. Every object must also be _thoroughly

dry_, otherwise dampness will arise and become condensed in small

drops upon the inner surface of the thin glass cover. This defect is

frequently met with in slides which have been mounted quickly; the

objects not being thoroughly dry when enclosed in the cell. Many of the

cheap slides are thus rendered worthless. Even with every care it is

not possible to get rid of this annoyance occasionally.

For the purpose of mounting opaque objects “dry,” _discs_ were at one

time very commonly made use of. These are circular pieces of cork,

leather, or other soft substance, from one-quarter to half inch in

diameter, blackened with varnish or covered with black paper, on which

the object is fixed by gum or some other adhesive substance. They are

usually pierced longitudinally by a strong pin, which serves for the

forceps to lay hold of when being placed under the microscope for

examination. Sometimes objects are affixed to both sides of the disc,

which is readily turned when under the object-glass. The advantage of

this method of mounting is the ease with which the disc may be moved,

and so present every part of the object to the eye save that by which

it is fastened to the disc. On this account it is often made use of

when some particular subject is undergoing investigation, as a number

of specimens may be placed upon the discs with very little labour,

displaying all the parts. But where exposure to the atmosphere or small

particles of dust will injure an object, no advantage which the discs

may possess should be considered, and an ordinary covered cell should

be employed. Small pill-boxes have been used, to the bottom of which a

piece of cork has been glued to afford a ground for the pin or other

mode of attachment; but this is liable to _some_ of the same faults as

the disc, and it would be unwise to use these for permanent objects.

Messrs. Smith and Beck have lately invented, and are now making, a

beautiful small apparatus, by means of which the disc supporting the

object can be worked with little or no trouble into any position that

may prove most convenient, whilst a perforated cylinder serves for the

reception of the discs when out of use, and fits into a case to protect

them from dust. A pair of forceps is made for the express purpose of

removing them from the case and placing them in the holder.

All dry objects, however, which are to be preserved should be mounted

on glass slides in one of the cells (described in Chapter I.) best

suited to them. Where the object is to be free from pressure, care

must be taken that the cell is deep enough to ensure this. When the

depth required is but small, it is often sufficient to omit the card,

leather, or other circles, and with the “turn-table” before described,

by means of a thick varnish and camel-hair pencil, to form a ring

of the desired depth; but should the varnish not be of sufficient

substance to give such “walls” at once, the first application may be

allowed to dry, and a second made upon it. A number of these may be

prepared at the same time, and laid by for use. When liquids are used

(see Chapter IV.), Dr. Carpenter recommends gold-size as a good varnish

for the purpose, and this may be used in “dry” mountings also. I have

used the asphaltum and india-rubber (mentioned in Chapter I.), and

found it to be everything I could wish. The cells, however, must be

_thoroughly dry_, and when they will bear the heat they should be baked

for an hour at least in a tolerably cool oven, by which treatment the

latter becomes a first-rate medium. All dry objects which will not bear

pressure must be firmly fastened to the slide, otherwise the necessary

movements very often injure them, by destroying the fine hairs, &c.

For this purpose thin varnishes are often used, and will serve well

enough for large objects, but many smaller ones are lost by adopting

this plan, as for a time, which may be deemed long enough to harden

the varnish, they exhibit no defect, but in a while a “wall” of the

plastic gum gathers around them, which refracts the light, and thus

leads the student to false conclusions. In all _finer_ work, where it

is necessary to use any method of fixing them to the slide, a solution

of common gum, with the addition of a few drops of glycerine (Chapter

I.), will be found to serve the purpose perfectly. It must, however,

be carefully filtered through blotting paper, otherwise the minute

particles in the solution interfere with the object, giving the slide a

dusty appearance when under the microscope.

When mounting an object in any of these cells, the glass must be

thoroughly cleaned, which may be done with a cambric handkerchief,

after the washing mentioned in Chapter I. _If the object be large_,

the point of a fine camel-hair pencil should be dipped into the gum

solution, and a minute quantity of the liquid deposited in the cell

where the object is to be placed, but not to cover a greater surface

than the object will totally hide from sight. This drop of gum must be

allowed to dry, which will take a few minutes. Breathe then upon it two

or three times, holding the slide not far from the mouth, which will

render the surface adhesive. Then draw a camel-hair pencil through the

lips, so as to moisten it slightly (when anything small will adhere to

it quite firmly enough), touch the object and place it upon the gum in

the desired position. This must be done immediately to ensure perfect

stability, otherwise the gum will become at least partially dry and

only retain the object imperfectly.

When, however, the objects are so minute that it would be impossible to

deposit atoms of gum small enough for each one to cover, a different

method of proceeding must be adopted. In this case a small portion of

the same gum solution should be placed upon the slide, and by means

of any small instrument--a long needle will serve the purpose very

well--spread over the surface which will be required. The quantity thus

extended will be very small, but by breathing upon it may be prevented

drying whilst being dispersed. This, like the forementioned, should be

then allowed to dry; and whilst the objects are being placed on the

prepared surface, breathing upon it as before will restore the power of

adherence.

When gum or other liquid cement has been used to fix the objects to

the glass, the thin covers must not be applied until the slide has

been _thoroughly dried_, and all fear of dampness arising from the use

of the solution done away with. Warmth may be safely applied for the

purpose, as objects fastened by this method are seldom, if ever, found

to be loosened by it. As objects are met with of every thickness, the

cells will be required of different depths. There is no difficulty in

accommodating ourselves in this--the deeper cells may be readily cut

out of thick leather, card, or other substance preferred (as mentioned

in Chapter I.). Cardboard is easily procured of almost any thickness;

but sometimes it is convenient to find a thinner substance even than

this. When thin glass is laid upon a drop of any liquid upon a slide,

every one must have observed how readily the liquid spreads betwixt

the two: just so when any thin varnish is used to surround an object

of little substance, excessive care is needed lest the varnish should

extend betwixt the cover and slide, and so render it worthless. The

slightest wall, however, prevents this from taking place, so that a

ring of common paper may be used, and serve a double purpose where the

objects require no deeper cell than this forms.

Many objects, however, are of such tenuity--as the leaves of many

mosses, some of the Diatomaceæ, scales of insects, &c.--that no cell is

requisite excepting that which is necessarily formed by the medium used

to attach the thin glass cover to the slide; and where the slide is

covered by the ornamental papers mentioned in Chapter I., and pressure

does not injure the object, even this is omitted, the thin glass being

kept in position by the cover; but slides mounted in this manner are

frequently injured by dampness, which soon condenses upon the inner

surfaces and interferes both with the object and the clearness of its

appearance.

The thin glass, then, is to be united to the slide, so as to form a

perfect protection from dust, dampness, or other injurious matter,

and yet allow a thoroughly distinct view of the object. This is to be

done by applying to the glass slide round the object some adhesive

substance, and with the forceps placing the thin glass cover (quite dry

and clean) upon it. A gentle pressure round the edge will then ensure

a perfect adhesion, and with ordinary care there will be little or no

danger of breakage. For this purpose gold-size is frequently used. The

asphalt and india-rubber varnish also will be found both durable and

serviceable. Whatever cement may be used, it is well to allow it to

become in some measure “fixed” and dried; but where no cell or “wall”

is upon the slide, this is _quite necessary_, otherwise the varnish

will be almost certain to extend, as before mentioned, and ruin the

object. It may be stated here that gold-size differs greatly in its

drying powers, according to its age, mode of preparation, &c. (Chapter

IV.)

Should any object be enclosed which requires to be kept flat during

the drying of the cement, it will be necessary to use some of the

contrivances mentioned in Chapter I.

When the slide is thus far advanced, there remains the “finishing”

only. Should the student, however, have no time to complete his work

at once, he may safely leave it at this stage until he has a number

of slides which he may finish at the same time. There are different

methods of doing this, some of which may be here described.

If ornamental papers are preferred, a small circle must be cut out

from the centre a little less than the thin glass which covers the

object. Another piece of coloured paper is made of the same size, and

a similar circle taken from its centre also, or both may be cut at the

same time. The slide is then covered round the edges with paper of any

plain colour, so that it may extend about one-eighth of an inch over

the glass on every side. The ornamental paper is then pasted on the

“object” surface of the glass, so that the circle shows the object as

nearly in the centre as possible, and covers the edges of the thin

glass. The other coloured paper is then affixed underneath with the

circle coinciding with that above. And here I may observe, that when

this method is used there is no necessity for the edges of the slide to

be “ground,” as all danger of scratching, &c., is done away with by the

paper cover.

Many now use paper covers, about one and a half inches long, on the

upper side of the slide only, with the centre cut out as before, with

no other purpose than that of hiding the edge of the thin glass where

it is united to the slide.

The method of “finishing,” however, which is mostly used at the present

time, is to lay a coating of varnish upon the edge of the thin glass,

and extend it some little way on the slide. When a black circle is

required, nothing serves the purpose better than the gold-size and

lamp-black, or the asphalt and india-rubber varnish, neither of which

is liable to chip; but when used for this, the latter should be rather

thinner, as before advised. Some of these varnishes are preferred of

different colours, which may be made by using the different kinds of

sealing-wax, as described in Chapter I.; but they are always liable

to the defects there mentioned. This circle cannot be made in any

other way but by one of those contrivances which have now centred in

Shadbolt’s turntable. A very little practice will enable the young

student to place his slide so that the circle may be uniform with the

edge of the thin glass.

The slide is now complete, except the addition of the name and any

other particulars which may be desirable. For this purpose one of the

methods described in Chapter I. must be employed.

Amongst the various classes of microscopic objects now receiving

general attention, the Diatomaceæ may be placed in a prominent

position. They afford endless opportunities of research, and some

very elaborate works have already been issued concerning them.

Professor Smith’s may be mentioned as one containing, perhaps, the

best illustrations. The young student may wish to know what a diatom

is. The “Micrographic Dictionary” gives the following definition:--“A

family of confervoid Algæ, of very peculiar character, consisting of

microscopic brittle organisms.” They are now looked upon by almost all

of our scientific men as belonging to the _vegetable_ kingdom, though

some few still assign them to the animal. They are almost invariably

exceedingly small, so that the unaided eye can perceive nothing on

a prepared slide of these organisms but minute dust. Each separate

portion, which is usually seen when mounted, is termed a “frustule,”

or “testule:” this consists of two similar parts, composed of silica,

between and sometimes around which is a mass of viscid matter called

the “endochrome.” They are found in almost every description of water,

according to the variety: some prefer sea-water, others fresh, and

many are seen nowhere but in that which is a mixture of both, as the

mouths of rivers, &c. Ditches, ponds, cisterns, and indeed almost every

_reservoir_, yield abundance of these forms. They are not, however,

confined to “present” life; but, owing to the almost indestructible

nature of their siliceous covering, they are found in a fossil state

in certain earths in great abundance, and are often termed “fossil

Infusoria.” Upon these frustules are generally to be seen lines,

or “markings,” of different degrees of minuteness, the delicacy of

which often serves the purpose of testing the defining power of the

object-glasses. Some of the frustules are triangular, others circular,

and, indeed, of almost every conceivable shape, many of them presenting

us with exquisitely beautiful designs.

The markings, however, are seldom seen well, if at all, until the

frustules are properly prepared, the different methods of accomplishing

which will be given a little further on.

The _collection_ of the fresh diatoms is so closely connected with

their _preservation_, that a few notes may be given upon it before we

pass on. For this purpose a number of small bottles must be provided,

which may be placed in a tin box, with a separate apartment for each,

so that all chance of breakage may be done away with. The diatoms

are generally of a light brown colour; and where they are observed

in the water, the bottle may be so placed, with the mouth closed by

the finger, that when the finger is withdrawn the water will rush in,

carrying the diatoms also. If they are seen upon plants, stones, or

any other substance, they may generally be detached and placed in the

bottle. When there is a green covering upon the surface of the water, a

great quantity of diatoms is usually found amongst it; as also upon the

surface of the mud in those ponds where they abound. In these cases, a

broad flat spoon will be found very useful, and one is now made with a

covering upon the broader portion of it to protect the enclosed matter

from being so readily carried off whilst bringing it to the surface

again. Where there is any depth of water, and the spoon will not reach

the surface of the mud, the bottle must be united to a long rod, and

being then carried through the upper portion with the mouth downwards,

no water will be received into it; but on reaching the spot required,

the bottle-mouth may be turned up, and thus become filled with what is

nearest.

From the stomachs of common fish--as the cod, sole, haddock, &c.--many

specimens of Diatomaceæ may be obtained, but especially from the crab,

oyster, mussel, and other shell-fish. Professor Smith states that from

these curious receptacles he has taken some with which he has not

elsewhere met. To remove them from any of the small shell-fish, it is

necessary to take the fish or stomach from the shell, and immerse it

in strong hot acid (nitric is the best) until the animal matter is

dissolved, when the residue must be washed and treated as the ordinary

Diatomaceæ hereinafter described.

Many diatoms are seen best when mounted in a dry state, the minute

markings becoming much more indistinct if immersed in liquid or balsam;

and for this reason those which are used as test objects are usually

mounted _dry_. Many kinds also are now prepared in this way, as opaque

objects to be examined with the lieberkuhn, and are exquisitely

beautiful. Others, however, are almost invariably mounted in balsam;

but as these will be again referred to in Chapter III., and require

the same treatment to fit them for the slide, it will not be out of

place to describe the cleaning and preparation of them here. As before

stated, there is much matter surrounding them which must be got rid of

before the “_siliceous_ covering” can be shown perfectly. As, however,

we may first wish to become acquainted in some degree with what we have

to do, it is well to take a small piece of _talc_, and place a few of

the diatoms upon it. This may be held over the flame of the spirit-lamp

until all the surrounding matter is burnt away, and a tolerable idea

may be thus obtained as to the quality of our treasure.

In some cases it is well to use this burning operation alone in

_mounting_ specimens of diatoms, when they may be placed in their

natural state upon the thin glass, burnt for awhile upon the platinum

plate, hereafter described, and mounted dry or in balsam.

In the preparation and cleaning of Diatomaceæ, there is little

satisfaction unless these operations have been successfully performed,

as a very small portion of foreign matter seriously interferes with

the object. The mode of preparing them varies even amongst the most

experienced. It will be found, therefore, most satisfactory to examine

the principal of these separately, although it may be at the risk of

some little repetition.

The method which is the most frequently made use of is the

following:--Place the “gathering” containing the Diatomaceæ in a small

glass or porcelain vessel, add strong nitric acid, and, by the aid

of Bunsen’s burner or spirit-lamp, boil for some minutes. From time

to time a drop of the mixture may be put upon a slide, and examined

under the microscope to see if all foreign matter be got rid of. When

the valves are clean, the vessel containing them must be filled with

water, and the whole left for an hour or two, so that all the diatoms

may settle perfectly. The liquid must then be poured off carefully,

or drained away by the aid of a syphon, so that none of the diatoms

are removed with it. Indeed, it is well to examine the liquid drained

off each time with the microscope, as the finer forms are frequently

lost in the washings. The vessel must then be refilled with pure

water, allowed to settle, and drained as before. This washing must be

repeated until a drop being placed upon a slide and evaporated leaves

no crystals. When it is desirable to preserve the diatoms in this state

before mounting (which process will be described in another place),

they may be placed in a small phial with a little distilled water.

There are many cases in which the above method will not effect a

_perfect_ cleansing, as certain substances with which the diatoms are

frequently mixed are not soluble in _nitric acid_. For this reason

the following method is resorted to:--Take a quantity of the matter

containing the Diatomaceæ and wash first with pure water, to get rid of

all the impurities possible. Allow this to settle perfectly and decant

the water. Add hydrochloric acid gradually, and when all effervescence

has subsided, boil for some minutes by aid of the lamp. When cool and

the particles have subsided, decant the hydrochloric and add nitric

acid. The boiling must then be repeated until a drop of the liquid when

placed under the microscope shows the valves or “frustules” clean.

After allowing the diatoms to settle, the acid must be decanted, and

pure water substituted. The washing must be repeated as in the former

process until all the remains of crystals or acid are removed, when the

specimens may be preserved in small phials as stated above.

Such are the usual modes of treating the Diatomaceæ, but there are

certain cases in which particular methods are required to give anything

like perfect results. Persons of great experience combine a variety of

treatments, and thus obtain better and more uniform specimens. Perhaps

it will be advantageous to give the young student the process adopted

by one of the most successful preparers of these objects; but I will

first state the different methods of mounting the cleaned diatoms dry:

how to employ Canada balsam and fluid in their preservation will be

elsewhere described.

It was before stated that the diatoms when cleansed might be preserved

in small phials of distilled water. When required for mounting, shake

the phial, and with a thin glass tube or rod take up a drop of the

fluid and spread it upon the surface of the slide in the desired

position. This must then be allowed to dry gradually, or by the aid

of the lamp if necessary, without being shaken or interfered with,

otherwise uniformity of dispersion will be prevented. When _thoroughly_

dry, a thin ring of one of the adhesive varnishes--gold-size will be

found as good as any--may be drawn round the diatoms, and allowed to

dry in a slight degree. The slide and thin glass cover should then be

warmed and the latter gently pressed upon the ring of varnish until the

adhesion all round is complete.

As some of the diatoms require object-glasses of extremely high power,

and, consequently, short focus, to show them, they must be as close to

the outer surface of the cover as possible. For this reason they are

sometimes placed upon the under side of the thin glass, as follows.

Clean the surfaces of the slide and cover, and with the rod or pipe

place the liquid containing the diatoms upon the thin glass, and dry as

before. Trace the ring to receive the cover upon the slide, and when

almost dry, warm both and proceed as above. Whichever of these methods

is employed, the outer ring of coloured varnish may be applied as

elsewhere described and the slide finished.

The diatoms are also sometimes mounted betwixt two thin glasses, as

described in Chapter I., so that the light by which they are examined

may receive as little interference as possible, and that an achromatic

condenser may be brought into focus under the slide.

Of the various modes of cleaning and mounting the Diatomaceæ, I believe

that the following may be safely recommended as affording results of

the very best quality. My friend, Mr. T. G. Rylands, gave it to me as

that which he prefers, and I can safely say that his numerous slides

are at least equal to any I have ever seen. I will give it just as I

received it from him, though there may be some little repetition of

what has been said elsewhere, as he does not appropriate any part of it

as his own. He says:--In this branch of mounting, general rules alone

can be laid down, because the gatherings may contain iron, lime, fine

silt, or vegetable matter under conditions for special treatment, and

consequently the first step should be to experiment on various kinds.

In gathering diatoms much labour is saved by judgment and care;

hence it is desirable to get acquainted with them in their growing

condition, so that when recognised upon the sands or other spots they

may be carefully removed by the aid of the spoon or small tin scoop

before described. When growing upon algæ or other plants, the plants

and diatoms together may be carried home, in which case they must be

simply drained and not washed or pressed, in order that the diatoms

be not lost. As it is always desirable to examine the gathering on

the ground, a “Gairdner’s hand microscope” with powers from 80 to 200

diameters will be found very useful. The best gatherings are those

which represent one species abundantly. Those which are mixed may

be rejected, unless they are seen to contain something valuable or

important, as the object should be not so much to supply microscopical

curiosities as to collect material which is available for the study of

nature.

The gathering when carried home should always be carefully examined

before anything is done with it; not only on account of the additional

information thus acquired, but also because it often happens that a

specimen should be mounted in fluid (see Chapter IV.) in the condition

in which it is gathered, as well as cleaned and mounted in balsam

(Chapter III.) and dry.

Where the gathering is taken from sand, the whole may be shaken up

in water as a preliminary operation, when much of the sand will

be separated by its own weight. The lime test, however, should be

applied, viz.--a small portion of hydrochloric acid, and if there be

effervescence it must be dissolved out by this means. From Algæ and

other weeds diatoms may be detached by agitating the whole together in

a weak solution of nitric acid--about one of pure acid to twenty or

thirty of water, as it must be sufficiently weak to free the diatoms

without destroying the matter to which they adhere. The diatoms

may then be separated by sifting through coarse muslin, which will

retain the Algæ, &c. The process of cleaning will vary according to

circumstances. Some gatherings require to be boiled only a few minutes

in nitric acid; but the more general plan where they are mixed with

organic or other foreign matter, is to boil them in pure sulphuric

acid until they cease to grow darker in colour (usually from a half

to one minute), and then to add, drop by drop to avoid explosions,

a cold saturated solution of chlorate of potash until the colour is

discharged, or, in case the colour does not disappear, the quantity

of the solution used is at least equal to that of the acid. This

operation is best performed in a wide-mouthed ordinary beaker glass,[B]

a test-tube being too narrow. The mixture whilst boiling should be

poured into thirty times its bulk of cold water, and the whole allowed

to subside. The fluid must then be carefully decanted and the vessel

re-supplied once or twice with pure water, so as to get rid of all the

acid. The gathering may then be transferred to a small boiling glass

or test-tube, and--the water being carefully decanted--boiled in the

smallest available quantity of nitric acid, and washed as before. This

last process has been found necessary from the frequent appearance of

minute crystals, which cannot otherwise be readily disposed of without

the loss of a considerable proportion of diatoms.

[B] These glasses are round, about six inches high, and usually

contain about eight ounces. They are rather wider at the

bottom, tapering gradually to the top, and may be generally

procured at the chemists, &c.

I may here mention that the washing glasses used by Mr. Rylands are

stoppered conical bottles varying in capacity from two ounces to one

quart; the conical form being employed to prevent the adherence of

anything to the side; they are “stoppered” to render them available in

the shaking process about to be described.

The gathering, freed from acid, is now put into two inches depth of

water, shaken vigorously for a minute or two, and allowed to subside

for half an hour, after which the turbid fluid must be carefully

decanted. This operation must be repeated until all the matter is

removed which will not settle in half an hour. The fluid removed should

be examined by a drop being put upon a slide, as in some cases very

light diatoms have been found to come off almost pure in one or more of

these earlier washings. The quantity of water and time of subsidence

given may be taken generally, but may require to be modified according

to circumstances and the judgment of the operator. By the repetition

and variation of this process--the _shaking_ being the most important

part--the gathering, if a _pure_ one, will be sufficiently clean. If,

however, it contains a variety of species and forms, it may require to

be divided into _different densities_.

In some cases, however, it is best to divide the gathering as a

_preliminary_ operation, which may be done by agitating it in a

quantity of water and decanting what does not readily subside. The

heavier and the lighter portions are then to be treated as two

separate boilings. But when the cleansing has been carried to the

above stage and this division is required, the plan must be somewhat

as follows:--The gathering must be shaken in a test-tube with six

inches of water, and then allowed to subside until one inch at the top

remains pure. About three inches are then to be carefully withdrawn by

a pipette, when the tube may be filled up and the operation repeated.

The three lower inches also may then be decanted and examined. The

gathering is thus divided into three portions, viz.--that which was

withdrawn by the pipette, that which remained floating in the lower

three inches of water in the tube, and that which had settled at the

bottom. An examination of these will inform the operator how to obtain

that particular density of gathering which he desires, and how far it

is worth while to refine this process of elutriation; for in cases of

necessity any one, or all three, of these densities may be operated

upon in the same way to separate a particular diatom.

As occasional aids, it may be remarked, that in some cases _liquor

ammoniæ_ may be used in place of water, as it often separates fine

dirt, which is not otherwise easily got rid of. Some fossil deposits

require to be treated with a boiling solution of carbonate of soda to

disintegrate them; but this operation requires great care, lest the

alkali should destroy the diatoms. Vegetable silicates also sometimes

require to be removed by a solution of carbonate of soda; but as the

frustules of the diatoms themselves are but _vegetable silica_, even

more care is required in this case. It may be well to mention, that

some diatoms are so imperfectly siliceous that they will not bear

_boiling_ in acid at all. Some of these may be allowed to stand in cold

nitric acid some time, whilst others of a smaller and more delicate

character should, when possible, be treated with distilled water alone.

We will now consider the mode of mounting the prepared diatoms, which,

if used dry (as described in this chapter), should be carefully washed

two or three times with the purest distilled water. In this branch,

as in every other, each collector gives preference to that method in

which he is an adept. Thus the diatoms may be placed on the under side

of the cover, to be as near to the object-glass as possible, or upon

the slide itself; and each plan has its advocates. Whichsoever of these

is used, nothing seems more simple to the novice than a tolerably

equal dispersion of the objects upon the slide or cover; but this is

by no means so readily accomplished, consequently I give Mr. Rylands’

method, as his slides are perfect in this respect also. He always

places the diatoms upon the thin glass cover. It is not sufficient,

as is frequently thought, to take a drop of liquid containing the

cleansed material and spread it upon the cover or slide, as without

some additional precaution that uniform and regular distribution of the

specimens is not obtained which is desirable. In order to effect this,

let a drop of the cleansed gathering be diluted sufficiently for the

purpose--how much must be determined in each case by experiment--and

let the covers to be mounted be cleaned and laid upon the brass plate.

(See Chapter I.) By means of a glass tube, about one-twelfth of an

inch in diameter, stopped by the wetted finger at the upper end, take

up as much of the diluted material as will form a moderately convex

drop extending over the whole cover. When all the covers required are

thus prepared, apply a lamp below the brass plate, and raise the

temperature to a point just short of boiling. By this means the covers

will be dried in a few minutes, and the specimens equally distributed

over the whole area. The spread of the fluid upon the covers is

facilitated by breathing upon them; and, to insure uniformity, care

must be taken to avoid shaking them whilst drying. The best plan is to

mount at least half a dozen at once.

Before mounting, Mr. Rylands always burns the diatoms upon the glass

at a dull red heat, whether they are used with balsam or dry. This

burning, he says, is not only an additional cleaning process, but

it effectually fixes the diatoms, and prevents them floating out if

mounted with balsam. The thinnest covers may be burnt without damage

if they are placed upon a small piece of platinum foil of the size

required, which should be about one-hundredth of an inch thick,

perfectly flat, and having three of its edges slightly bent over, so as

to prevent its warping with the heat. The small flame of a spirit-lamp,

or, where there is gas, a Bunsen’s burner, may be employed. The cover

should be shaded from direct daylight, that the action of the flame

may be observed more perfectly. Care must then be taken to raise the

temperature only to the dull red heat before mentioned. The cover will

then be in a fit state for mounting as required.

It has been stated in another place that it is assumed the operator is

not mounting diatoms simply as microscopic objects, but as instructive

specimens. It is not, therefore, sufficient to take a single slide as

all that is required, but to have the same diatom prepared in as many

ways as possible. The following are the principal:--

1. Mounted crude in fluid (see Chapter IV.).

2. Burnt crude upon the cover, and mounted dry or in balsam (as

before mentioned).

3. Mounted dry or in balsam (see Chapter III.), after the

cleansing process already described.

I will here give Mr. Rylands’ method of mounting them _dry_, the fluid

and balsam preparations being noticed in their respective chapters.

The slide with the ring of asphalt, or black varnish, should have been

prepared some weeks previously, in order to allow it to dry thoroughly.

When required, it must be held over the spirit-lamp or Bunsen’s burner

until the ring of varnish is softened. The burnt cover, having been

heated at the same time, must then be taken in the forceps and pressed

upon the softened varnish until it adheres all round. When cold, an

outer ring of asphalt completes the slide.

Such is the method which my friend Mr. T. G. Rylands employs in

the preparation of diatoms for the microscope. I have said enough

concerning his results. It is to be feared, however, that to some these

several modes of operation may appear lengthy and complicated; but if

read carefully, and the experiments tried, they will be found simple

enough in practice, and to occupy much less time than an intelligible

description would lead the novice to believe necessary.

One of the most fertile as well as the most curious magazines of

Diatomaceæ is _guano_. The siliceous forms contained therein have been

devoured by sea-birds and passed through the stomach uninjured, and

after lying for ages may be cleaned and classified. Many of these are

not elsewhere met with, so that the student who is desirous to enter

into the study of Diatomaceæ must be instructed as to the best mode

of obtaining them from this source. The particulars to be observed

so closely resemble those before mentioned in the treatment of the

ordinary diatoms, that it will be sufficiently explicit to give the

outlines of the process. The guano must be first washed in pure water,

allowed to subside perfectly, and the liquid then poured off. This must

be repeated until the top fluid is clear, and care taken not to decant

the liquid until perfect subsidence has taken place. The deposit must

then be treated with hydrochloric acid with a gentle heat for an hour

or two, adding a little fresh acid at intervals as long as it excites

any effervescence After this nitric acid must be substituted for the

hydrochloric, and the heat kept up to almost boiling-point for another

hour at least, adding a little fresh acid as before. When this ceases

to act, the deposit must be allowed to settle perfectly and the acid

poured off. All traces of the acid must now be washed away with pure

water, when the remains will be Diatomaceæ, the sand contained in the

guano, and a few other forms. Some of these may be mounted dry, as

before mentioned, but the greater portion should be put up in Canada

balsam as described in Chapter III.

Such is the ordinary method for the treatment of guano; but Mr.

Rylands’ mode of proceeding with ordinary Diatomaceæ (before given)

will be found equally successful with these deposits.

The fossil Infusoria (as they were formerly called) are now termed

Diatomaceæ, and are found in various parts of the world--“Bermuda

earth,” “Berg-mehl” from Norway, deposit from Mourne Mountain in

Ireland, &c. They are found in immense quantities, and afford the

microscopist innumerable objects. The same treatment as that usually

employed for the Diatomaceæ must be followed with these deposits, but

as they are sometimes obtained in hard masses, disintegration is first

necessary. To effect this, they are usually boiled for a short time in

diluted _liquor potassæ_, which will soon cause the mass to fall into a

mud-like deposit. Water must then be immediately added, in order that

all further action of the _liquor potassæ_ may be stopped, otherwise

the objects searched for will be dissolved. For this reason it is

necessary to understand what substance is being dealt with, because

some deposits are much finer and acted upon more readily than others.

In mounting these objects, some are so delicate that they are almost

invisible when balsam is used with them; they are therefore usually

mounted _dry_. Others, however, are much coarser, and may be mounted in

balsam like the Diatomaceæ mentioned in Chapter III.

The common Infusoria cannot be mounted dry with any great success,

though a few may be placed upon the glass slide and allowed to dry

naturally, when their characters will be very well shown. To obtain

anything like a natural appearance, they must be put up in fluid as in

Chapter IV.

Next to the Diatomaceæ, no class of microscopic objects has been more

looked into of late than the Foraminifera. These animals are almost

all marine, having a jelly-like body enclosed in one or more chambers

of shell generally composed of carbonate of lime. The shells are made

with minute orifices, through which the pseudopodia (false feet) are

extended by which the animal is enabled to lay hold of anything and

draw itself along. From the possession of these orifices they derive

their name, as _foramen_ means a door or opening. They have been found

in every depth of sea hitherto sounded, each depth being abundant in

certain species; the lowest beds containing the greatest number of

specimens, though with less variation of kinds. In chalk they are found

in a fossil state, and may readily be shown (see Chapter III.); in

limestone and other hard stones they are abundant, and some mountains

are composed principally of these shells.

The methods of obtaining Foraminifera are various. Many may be found

upon seaweeds, which should always be examined as soon as possible

after gathering. They are found in masses upon some coasts where the

waves have carried and left them; but they are to be found the most

abundantly in sand or mud dredged from the bottom of the sea. They

must, however, be cleansed and separated from the mass of impurity

with which they are usually mixed. This may be done in various ways,

according to the nature of the accompanying matter. If sand alone, as

is frequently the case, the whole mass must be _thoroughly_ dried, and

then stirred up in clean water. The sand will soon subside by its own

weight, but the chambers of the Foraminifera, being filled with air,

will float upon the surface, and may be skimmed off. There is, however,

one objection to this mode of proceeding--some of these objects are

so minute, the chambers containing comparatively so small a quantity

of air, that they sink and are cast away with the refuse sand. On this

account it is preferable to take the trouble of searching certain

soundings under the microscope, using the camel-hair pencil, or some

other contrivance before mentioned, to extract those objects which

are required. To clean the Foraminifera, Professor Williamson advises

to transfer the specimens to an evaporating dish containing a weak

solution of caustic potash. This must be boiled for “some moments,”

when the organic matter will be entirely dissolved, and the calcareous

shells left free from impurity. They must now be well washed in water,

so that all alkaline matter may be entirely got rid of.

If the specimens are in _mud_, we must proceed in a different

way:--Stir up the whole mass in water, and allow it to stand until the

heavier portion has sunk to the bottom; the water may then be poured

off and examined to see if there are any objects contained in it.

This process must be repeated until the water comes off quite clear,

when (if the search is for Foraminifera only) the solution of caustic

potash may be used as before mentioned. However the soundings, &c., are

cleaned, it is necessary to assort them under the microscope with the

camel-hair pencil or other contrivance, as it is impossible to obtain

them fit for mounting without undergoing this process.

The sea soundings taken by order of Government are drawn from the

bottom in a kind of apparatus ingeniously made for the purpose, and

the sand, mud, &c., are brought up in their original state. Common

soundings, however, are taken by lowering a heavy piece of lead coated

with tallow, which consequently brings up a small portion of the

matter from the bottom. Mr. George Mosley, the late Secretary of the

Manchester Microscopic Society, obtained numbers of the “scrapings”

from the sounding leads. To make any use of these it is, of course,

necessary to free them from all traces of the tallow. Mr. Dancer

places the sounding in a basin and pours boiling water upon it, which

causes the melted grease to rise to the surface. When cold, this may

be removed, and the water carefully decanted. The operation may be

repeated until no grease appears, when the water may be withdrawn

and _liquor ammoniæ_ used, which will form a soapy solution with any

remaining grease. This must be treated with hot water for the final

washing. Care must be taken lest the finer forms be carried away in

decanting the washing liquid. Should it be wished to make certain as

to this point, each washing should be examined under the microscope.

In some cases the process of Mr. Dancer will prove sufficient. Mr.

Dale, however, gives a method of accomplishing the same result, which

is much more readily completed; and as the results cannot be found

fault with, I will here give it in full:--It is now well known that one

of the products obtained from the naphtha of coal-tar is a volatile,

oily substance, termed _benzole_ (or, by French chemists, _benzine_),

whose boiling-point, when pure, is about 180° Fahrenheit, which is a

perfect solvent for fatty substances. In a capsule, previously warmed

on a sand-bath, Mr. Dale mixes with the tallow soundings benzole,

whose boiling-point may be about 200°, until sufficiently diluted so

as to run freely, pressing the lumps with a glass rod until thoroughly

mingled; the solution and its contents are then poured into a paper

filter, placed in a glass funnel; the capsule is again washed with

benzole, until the whole of the gritty particles are removed into

the filter. A washing-bottle is then supplied with benzole, and the

contents of the filter washed to the bottom until that liquid passes

off pure, which may be tested by placing a drop from the point of the

funnel on a warm slip of glass or bright platinum, when, if pure,

the benzole will evaporate without residue or tarnish; if grease be

present, the washings must be continued until they are free from it.

After rinsing through _weak_ acid, or alcohol, for final purification,

the calcareous forms will be ready for mounting.

The filter and its contents may be left to dry spontaneously, when the

latter can be examined by the microscope. Should time be an object,

rapid drying may be effected by any of the usual methods; one of

which, recommended by Mr. Dale, is to blow a stream of hot air through

a glass tube held in the flame of a Bunsen’s burner. The lower the

boiling-point of the benzole, the more readily can the specimens be

freed from it. A commoner quality may be used, but it is more difficult

to dry afterwards.

Pure benzole being costly, this may appear an expensive process; but,

with the exception of a trifling loss by evaporation, the whole may be

recovered by simple distillation. The mixture of tallow and benzole

being placed in a retort in a hot-water, a steam, or a sand bath, the

benzole will pass into the receiver, and the tallow or other impurities

will remain in the retort. When the whole of the benzole has distilled

over, which is ascertained by its ceasing to drop from the condenser,

the heat is withdrawn and the retort allowed to cool before the

addition of fresh material. Half a dozen to a dozen filters, each with

its specimen, can be in process at the same time; and the distillation

of the recovered benzole progresses as quickly as the filtration, which

was practically proved on the occasion named. Great caution in the use

of benzole is to be taken in the approach of lights to the inflammable

vapour.

After the Foraminifera and calcareous forms have been removed, the

residue may be treated with acids and levigation in the usual manner,

to obtain siliceous forms and discs, if there are any present; but to

facilitate their deposition, and to avoid the loss of any minute atoms

suspended in the washings, I would suggest the use of filtration. The

conical filter is unsuitable, as the particles would spread over too

great a surface of paper; but glass tubes open at both ends (such as

broken test-tubes) will be found to answer, the broad end covered

with filtering paper, and over that a slip of muslin tied on with a

thread to facilitate the passage of the water and prevent the risk of

breaking the paper. Suspend the tube over a suitable vessel through a

hole cut in thin wood or cardboard, pour in the washings which can be

thus filtered and then dried. The cloth must be carefully removed, the

paper cut round the edges of the tube, and the diatoms on the paper

disc may be removed by a camel-hair pencil or otherwise, ready for

mounting. Thus many objects may be preserved which would be either

washed away or only be obtained by a more tedious process.

Such is Mr. Dale’s method of cleaning the soundings from the tallow,

and as it thoroughly accomplishes its end, and is alike effective

and not injurious to Foraminifera and diatoms, it may be safely

recommended. The weak solution of caustic potash before advised for

Foraminifera, must not be used where it is desired to preserve the

diatoms, as they would certainly be injured, or destroyed altogether,

if this agent were employed.

In fixing the Foraminifera upon the slide, no better plan can be

followed than the “dry cells” and gum recommended in the early parts

of this chapter. Owing to their thickness and composition, most of

them are opaque objects only; but they are exquisitely beautiful,

and require no particular care, except in allowing the cell, &c., to

be perfectly dry, when the cover is placed upon it, or the damp will

certainly become condensed upon the inner side, and the examination

seriously interfered with.

Many of the Foraminifera require cutting into sections if it is

wished to examine the internal structure, &c.,--“decalcifying” is

also desirable in some cases;--both of these processes will be found

described at length in the chapter on Sections and Dissection.

When more than one specimen of some particular shell is obtained, it is

better to place them upon the slide in different positions, so as to

show as much of the structure as possible. I will conclude this subject

by quoting a passage from T. Rymer Jones:--“It is, therefore, by no

means sufficient to treat these shells as ordinary objects by simply

laying them on a glass slide, so as to see them only from one or two

points of view; they must be carefully examined in every direction,

for such is the diversity of form that nothing short of this will be

at all satisfactory. For this purpose, they should be attached to the

point of a fine needle, so that they may be turned in any direction,

and examined by reflected light condensed upon them by means of a

lens or side reflector. In many of the thick-shelled species it will

be necessary to grind them down on a hone [see Chapter V.] before the

number and arrangement of the internal chambers is discernible; and in

order to investigate satisfactorily the minutiæ of their structure, a

variety of sections, made in various ways, is indispensable.”

Plants afford an almost inexhaustible treasury for the microscope,

and many of them show their beauties best when mounted dry. When any

of these also are to be mounted, care must be taken that they are

thoroughly dry, otherwise the damp will certainly arise in the cell,

and injure the object; and it may be here mentioned that long after a

leaf has every appearance of dryness, the interior is still damp, and

no way can be recommended of getting rid of this by any quicker process

than that effected by keeping them in a warm room, as many leaves,

&c., are utterly spoiled by using a hot iron or other contrivance. The

safest way is to press them gently betwixt blotting-paper, which may

be removed and dried at short intervals; and though this may appear a

tedious operation, it is a _safe_ one.

On the surface of the leaves, hairs and scales of various and very

beautiful forms are found, most of which display their beauties best

when removed from the leaf, and used with the polarizer. These will be

noticed in another place; but a portion of the leaf should always be

prepared in its natural form, to show the arrangement of the hair or

scales upon it; which must almost invariably be mounted dry when used

for this purpose. Many of them require very delicate handling. The

_epidermis_, or, as it is by some termed, the _cuticle_, is the outer

skin which lies upon the surface of the leaves and other parts of most

plants. This is composed of cells closely connected, often bearing

the appearance of a rude network. In many plants, by scraping up the

surface of the leaf, a thin coating is detached, which may be torn off

by taking hold of it with the forceps. The piece may then be washed and

floated upon a glass slide, where, on drying, it will be firmly fixed,

and may usually be mounted dry. Amongst the most beautiful and easily

prepared of these may be mentioned the petal of the geranium, the cells

of which are well defined and amongst the most interesting.

Closely connected with the leaves are the ANTHERS and POLLEN, of

which a great number are beautiful and interesting subjects for the

microscopist.

The mallow tribe will furnish some exquisite objects, bearing the

appearance of masses of costly jewels. These are usually dried with

pressure, but the natural form may be more accurately preserved by

allowing them to dry as they are taken from the flower, with no

interference except thoroughly protecting them from all dust. Sometimes

the anther is divided, so that the cell required to receive them may

be of as little depth as possible. The common mallow is a beautiful

object, but I think the lavatera is a better, as it shows the pollen

chambers well, when dried unpressed. The pollen is often set alone, and

is well worth the trouble, as it then admits of more close examination.

Often it is convenient to have the _anther and pollen_ as seen in

nature on one slide, and the _pollen_ alone upon another. The former

should be taken from the flowers before their full development is

attained, as if overgrown they lose much of their beauty. Some pollens

are naturally so dark that it is necessary to mount them in Canada

balsam or fluid, as described in other places; but they are better

mounted dry when they are not too opaque.

Here, too, we may also mention the SEEDS of many plants as most

interesting, and some of them very beautiful, objects, requiring for

the greater part but a low power to show them. Most of these are to be

mounted dry, as opaque objects, in cells suited to them, but some are

best seen in balsam, and will be mentioned in Chapter III.

The CORALLINES, many of which are found on almost every coast, afford

some very valuable objects for the microscope. They must be well washed

when first procured, to get rid of all the salts in the sea-water,

dried and mounted in cells deep enough to protect them from all danger

of pressure, as some of them are exceedingly fragile. The white ivory

appearance which some of them present is given to them by an even

covering of carbonate of lime; and should it be desired to examine the

structure of these more closely, it may be accomplished by keeping them

for some time in vinegar or dilute muriatic acid, which will remove the

lime and allow of the substance being sliced in the same way as other

Algæ. (“Micrographic Dictionary,” p. 183.)

THE SCALES OF INSECTS.--The fine dust upon the wings of moths and

butterflies, which is so readily removed when handled carelessly,

is what is generally called _scales_. To these the wing owes the

magnificent colours which so often are seen upon it; every particle

being what may be termed a distinct flat feather. How these are placed

(somewhat like tiles upon a roof) may be easily seen in the wing of

any butterfly, a few being removed to aid the investigation. The form

of them is usually that of the “battledore” with which the common game

is played, but the handle or base of the scale is often short, and the

broad part varies in proportionate length and breadth in different

specimens. The markings upon these also vary, some being mostly

composed of lines running from the base to the apex, others reminding

us of network--bead-like spots only are seen in some--indeed, almost

endless changes are found amongst them. These scales are not confined

to butterflies and moths, nor indeed to the _wings_ of insects. The

different gnats supply some most beautiful specimens, not only from

the wings, but also from the proboscis, &c.; whilst from still more

minute insects, as the podura, scales are taken which were at one time

esteemed as a most delicate test. The gorgeous colours which the

diamond beetles also show when under the microscope are produced by

light reflected from minute scales with which the insects are covered.

In mounting these objects for the microscope it is well to have the

part of the insect from which the scales are usually taken as a

separate slide, so that the natural arrangement of them may be seen.

This is easily accomplished with the wings of butterflies, gnats, &c.;

as they require no extraordinary care. In mounting the _scales_ they

may be placed upon slides, by passing the wings over the surface, or

by gently scraping the wing upon the slide, when they must be covered

with the thin glass. Of course, the extreme tenuity of these objects

does away with the necessity of any cell excepting that formed by the

gold-size or other cement used to attach the cover. The scales of the

podura should be placed upon the slide in a somewhat different manner.

This insect is without wings, and is no longer than the common flea. It

is often found amongst the sawdust in wine-cellars, continually leaping

about by the aid of its tail, which is bent underneath its body. Dr.

Carpenter says:--“Poduræ may be obtained by sprinkling a little oatmeal

on a piece of black paper near their haunts; and after leaving it there

for a few hours, removing it carefully to a large glazed basin, so

that, when they leap from the paper (as they will when brought to the

light), they may fall into the basin, and may thus separate themselves

from the meal. The best way of obtaining their scales, is to confine

several of them together beneath a wine glass inverted upon a piece of

fine smooth paper; for the scales will become detached by their leaps

against the glass, and will fall upon the paper.” These scales are

removed to the slide, and mounted as those from the gnats, &c. When the

podura has been caught without the aid of the meal, it may be placed

upon the slide, under a test-tube, or by any other mode of confinement,

and thus save the trouble of transfer from the paper before mentioned.

Another method is to seize the insect by the leg with the forceps and

drag it across the slide, when a sufficient quantity of scales will

probably be left upon it.

These scales are usually mounted “dry;” but Hogg recommends the use of

Canada balsam (Chapter III.) as rendering their structure more definite

when illuminated with Wenham’s parabolic reflector. Some advise other

methods, which will be mentioned in Chapter IV. As most _insects_

when undissected are mounted in Canada balsam, the different modes of

treatment which they require will be stated in another place.

In mounting blood of any kind to show the corpuscles, or, as they are

often called, _globules_, which are round or oval discs, it is but

necessary to cover the slide on the spot required with a coating as

thin as possible and allow it to dry before covering with the thin

glass. There is a slight contraction in the globules when dried, but

not enough to injure them for the microscope. The shape of these varies

in different classes of animals, but the size varies much more, some

being many times as large as others.--Some of the _larvæ skins_ are

beautiful objects; but, like many sections of animal and other fragile

matter, are difficult to extend upon the slide. This difficulty is

easily overcome by floating the thin object in clear water, immersing

the slide and when the object is evenly spread gently lifting it. Allow

it then to dry by slightly raising one end of the slide to aid the

drainage, and cover with the thin glass as other objects. The tails and

fins of many small fish may be mounted in a similar manner, and are

well worth the trouble.

A few objects which are best shown by mounting _dry_ may be here

mentioned as a slight guide to the beginner, though some of them have

been before noticed;--many of the Foraminifera as elsewhere described.

Some _crystals_ are soluble in almost any fluid or balsam, and should

be mounted _dry_; a few, however, deliquesce or effloresce, which

renders them worthless as microscopic objects.

The wings of butterflies and gnats, as before noticed, afford many

specimens wherewith to supply the cabinet of the young student. A

great variety also may be found amongst the ferns; indeed, these alone

will afford the student occupation for a long time. On the under-side

of the leaves are the reservoirs for the “spores,” which in many

instances somewhat resemble green velvet, and are arranged in stripes,

round masses, and other forms. The spores are usually covered with a

thin skin termed the _indusium_, which is curiously marked in some

specimens, often very like pollen-grains. The manner in which these

spores with all their accompaniments are arranged, their changes and

developments afford almost endless subjects for study; different ferns

presenting us with many variations in this respect totally invisible

without the aid of the microscope. The hymenophyllums (of which two

only belong to England) are particularly interesting, and the structure

of the leaves when dried makes them beautiful objects, often requiring

no balsam to aid their transparency. Portions of the _fronds_ of ferns

should be mounted as opaque objects, after having been dried between

blotting paper, when they are not injured by pressure; but care must

be taken to gather them at the right time, as they do not show their

beauty before they are ripe, and if over-ripe the arrangement of the

spores, &c., is altered. The spores may be mounted as separate objects

in the same manner as pollen, before-mentioned, and are exquisitely

beautiful when viewed with a tolerably high power. The number of

foreign ferns now cultivated in this country has greatly widened the

field for research in this direction; and it may also be mentioned

that the under-sides of many are found to be covered with “scales” of

very beautiful forms. A small piece of the frond of one of these may

be mounted in its natural state, but the removal of the “scales” for

examination by polarized light will be described in another place. The

mosses also are quite a little world, requiring but a low power to show

their beauties. The leaves are of various forms, some of which resemble

beautiful net-work; the “urns” or reservoirs for the spores, however,

are perhaps the most interesting parts of these objects, as also of

the “liverworts” which are closely allied to the mosses. These “urns”

are generally covered by lids, which fall off when the fruit is ripe.

At this period they are well fitted for the microscope. The common

screw-moss may be found in great abundance, and shows this denudation

of the spores very perfectly. Many of these may be easily dried without

much injury, but they should also be examined in their natural state.

The student should not omit from his cabinet a leaf of the nettle and

the allied foreign species, the mystery of which the microscope will

make plain. The hairs or stings may also be removed, and viewed with a

higher power than when on the leaf, being so transparent as to require

no balsam or other preservative.

There are few more interesting objects than the _raphides_ or

_plant-crystals_. These are far from being rare, but in some plants

they are very minute, and consequently require care in the mounting,

as well as a high magnifying power to render them visible; in others

they are so large that about twenty-five of them placed point to point

would reach one inch. Some of these crystals are long and comparatively

very thin, which suggested the name (_raphis_, a needle); others are

star-like, with long and slender rays; while others again are of a

somewhat similar form, each ray being solid and short. If the stem

of rhubarb, or almost any of the hyacinth tribe, be bruised, so that

the “juice” may flow upon the slide, in all probability some of these

crystals will be found in the fluid. To obtain them clean, they must be

freed from all vegetable matter by maceration. After this they must be

thoroughly washed and mounted “dry.” They are also good _polarizing_

objects, giving brilliant colours; but when used for this purpose

they must be mounted as described in Chapter III. A few plants which

contain them may be mentioned here. The Cactaceæ are very prolific; the

orchids, geraniums, tulips, and the outer coating of the onion, furnish

the more unusual forms.

The Fungi are generally looked upon as a very difficult class of

objects to deal with, but amongst them some of the most available may

be found. The forms of many are very beautiful, but are so minute

as to require a high magnifying power to show them. The mould which

forms on many substances is a fungus, and in some cases may be dried

and preserved in its natural state. A friend of mine brought me a

rose-bush completely covered with a white blight. This was found to be

a fungus, which required a high magnifying power to show it. Being a

very interesting object, it was desirable to preserve it, and this was

perfectly effected without injury to the form by simply drying the leaf

in a room usually occupied. Amongst the fungi are many objects well

worth looking for, one of which is the _Diachæa elegans_. This, the

only species, says the “Micrographic Dictionary,” is found in England

upon the living leaves of the lily-of-the-valley, &c. These little

plants grow in masses, reminding one of mould, to a height of a quarter

of an inch, and each “stem” is covered with a sheath, in shape somewhat

like an elongated thimble. When ripe the sheath falls off and reveals

the same shaped column, made up of beautifully fine net-work, with the

spores lying here and there. This dries well, and is a good object for

the middle powers. Amongst the fungi the blights of wheat and of other

articles of food may be included. Many of them may be mounted “dry;”

others, however, cannot be well preserved except in liquids, and will

be referred to in Chapter IV. Amongst the zoophytes and sea-mats,

commonly called “sea-weeds,” may be found many very interesting objects

to be mounted “dry.” When this mode of preservation is used, it is

necessary that all the sea-salt be thoroughly washed from them. As they

are, however, most frequently mounted in balsam or liquid, they will be

more fully noticed in other places.

The _scales of fishes_ are generally mounted “dry” when used as

ordinary objects; but for polarized light, balsam or liquid must be

used, as noticed in Chapter III. The variety and beauty of these are

quite surprising to the novice. It is also very interesting to procure

the skin of the fish when possible, and mount it on a separate slide to

show how the scales are arranged. The sole is one of the most unusual

forms, the projecting end of each scale being covered with spines,

which radiate from a common centre, while those at the extremity are

carried out somewhat resembling the rays of a star. One of the skates

has a spine projecting from the centre of each scale, which is a very

curious opaque object, especially when the skin is mounted in the

manner described. The perch, roach, minnow, and others of the common

fishes give the student good objects for his cabinet, and may be

procured without difficulty.

Insects which are very transparent, or have the “metallic lustre”

with which any medium would interfere, are mounted “dry.” The

diamond-beetle, before mentioned, is a splendid example of this;

the back is generally used, but the legs, showing the curious feet,

are very interesting objects. Indeed, amongst the legs and feet of

insects there is a wide field of interest. When they are of a “horny”

nature, it is best to dry them in any form preferred, but to use no

pressure; when, however, they are wanted flat, so as to show the feet,

&c., extended, they must be dried with a gentle pressure betwixt

blotting-paper if possible. But this will be treated more fully in

Chapter III.

The _eyes of insects_ are sometimes allowed to dry in their natural

shape, and mounted as opaque objects; but generally they are used as

transparencies in balsam or liquid, so the description of the treatment

which they require will be deferred to Chapter III.

Hairs, when not too dark, are sometimes transparent enough when mounted

dry, but are usually mounted in balsam. These also will be more fully

noticed in another place.

These are a few of the objects which are often mounted dry, but some

of them should be shown in balsam or liquid also, and there is much

difference of opinion as to the best way of preserving others. This,

however, is explained by the transparency which the balsam gives

interfering with one property of the object and yet developing another

which would have remained invisible if preserved dry. The only method

of overcoming this difficulty is to keep the object mounted in both

ways, which is comparatively little trouble.

I may here mention that many prefer the lieberkuhn for the illumination

of opaque objects; and a good background is gained by putting upon the

under side of the slide, immediately beneath the object, a spot of

black varnish, which does not interfere materially with the light.

CHAPTER III.

MOUNTING IN CANADA BALSAM.

The nature and use of this substance has been before spoken of, so the

method of working with it may be at once described.

Perfect dryness of the objects is, if possible, more necessary in this

mode of mounting than any other, as dampness remaining in the object

will assuredly cause a cloudiness to make its appearance in a short

time after it is fixed. Where pressure does not injure the specimens,

they are most successfully treated when first dried betwixt the leaves

of a book, or in any other way which may prove most convenient, as

noticed in Chapter II.

Before describing the methods of proceeding with any particular

objects, general rules may be given which should be observed in order

to succeed in this branch of mounting.