If the microscopist should follow up his work properly, and make

sketches of every object which he places under the microscope, he cannot

do better than use the camera-lucida, a neat little instrument, which is

fitted into the eye-piece of the microscope. Dr. Beale’s neutral glass

is as efficacious in careful hands, and only costs a fourth of the sum.

This instrument cannot be applied to the ten and twenty shilling

microscopes, as it requires that the tube should be perfectly

horizontal. The method of using it is simple enough.

After fixing the object and getting the right focus, set the instrument

horizontally, and arrange the light so that the object is well

illuminated, and its lines quite clear and well defined. Now remove the

cap of the eye-piece, and fix the camera-lucida in its stead. Lay a

drawing-pad on the table under the camera-lucida, look through the

square opening (or, if you use Mr. Beale’s glass, look through the

neutral glass), and you will see the object apparently projected on the

paper. We say apparently, because in reality the image is not thrown on

the paper at all, but on the camera, and the eye refers it to the paper,

as being the nearest object. In fact, the principle on which this

camera-lucida is arranged is exactly that of the Polytechnic ghost,

which appears to be in one place, whereas it is in another.

Now take a pencil, cut it to a very fine point, and trace the outline of

the object on the paper. At first you will think this to be an

impracticable task, for the point of the pencil will totally vanish.

Soon, however, the eye will so adjust itself as to see the pencil and

the object perfectly well, and by a little practice the observer will be

able to sketch every object as rapidly and firmly as if he were copying

a drawing, by means of tracing-paper. The neutral glass is perhaps to be

preferred to the camera-lucida, as it is learned more easily, and gives

less trouble than that instrument. Its cost is five shillings.

After you have practised yourself well in the handling of the

microscope, your ambition will take another step, and lead you to the

preparation of permanent objects. In order to set yourself up with the

needful apparatus, you will have to disburse about five shillings. A

small spirit-lamp will cost eighteenpence, and a small bottle of Canada

balsam, another of asphalte varnish, and another of Dean’s gelatine,

will make about eighteenpence or two shillings more. A few pence will

purchase a sheet or two of ornamental paper, and a few more a flat plate

of brass or copper, about five inches by three. The rest of the five

shillings may be expended in “slides” and thin glass, cut square.

Slides are merely slips of glass, three inches in length by one in

width, and the thin glass is used for laying upon the objects and

defending them from dust. We advise the square glass, because it

scarcely costs one quarter as much as the round glass, and is equally

effective when properly managed. There are several methods of “putting

up” preparations--namely, dry, in Canada balsam, in gelatine, and in

cells. We will take them in their order.

The simplest plan is, of course, the “dry” mode. Suppose that you want

to preserve a tiny piece of down, or the scales from a butterfly’s wing.

First wash all the slides and glasses well, by dipping them into a

strong solution of soda, and then into hot water, in order to get rid of

grease, taking care never to touch them with the hand, but to take them

out of the water with the forceps. This can be done at any time, and the

glasses carefully wrapped up and placed in a box ready for use.

You now select one of the slides, and lay the object exactly in its

centre. If very minute objects are used, they must be examined in order

to see whether they are properly disposed. The next process is, to take

one of the thin glasses with the microscope, and lay it very carefully

over the object. Then cut a piece of ornamental paper, about two inches

long and seven-eighths of an inch in width; cut or punch a circular

piece out of its centre, damp it well, and cover the wrong side

slightly, but completely, with paste. Lay it on the slide, so that the

centre of the hole shall coincide with that of the object, work it down

neatly with the fingers, and it will hold the square piece of thin

glass, which is technically called the “cover,” in its place. Watch it

occasionally as it dries, and be ready to press down any part of the

paper that may start up. Write, with ink, the name of the object on the

end of the slide.

When you have made a dozen or two of these preparations, it will be

time to letter and index them. On each slide paste a slip of white

paper, and on the paper write a brief notice of the object, thus--

+---------------+

| SCALES. |

| |

| D. HEAD MOTH. |

+---------------+

Then scratch with a bit of flint, or with a writing-diamond, if you have

one, a number on the end of the slide, and have a note-book with a

corresponding number opposite to which you enter the description at a

fuller length, thus:--

18--Scales of Death’s Head Moth (_Acherontia Atropos_), from centre

of under-surface of right fore wing. Dry. June 4, 1864. +

The cross signifies that you prepared the object yourself, and the

reason for adding the date is, that in after years you will have a

valuable guide as to the durability of your preparations. If the

specimen has been purchased or presented, always add the name of the

seller or donor, as well as the date. These precautions may seem to be

needlessly minute, but we have so often seen whole sets of valuable

preparations rendered useless for want of ticketing, that we cannot too

strongly impress on our readers the necessity for the note-book as well

as the label, the one acting as a check upon the other. When the label

has been affixed, and the details transferred to the note-book, the ink

may be washed off the end of the slide.

There is another convenient method of putting up the elytra of beetles,

parts of various insects, mosses, minute shells, and similar objects.

Take a common pill-box of the smallest size, and cut a little cylinder

of cork, that will nearly, but not quite, equal the height of the box,

and fasten one end to the bottom of the box with glue. Now blacken the

interior of the box and the cork cylinder. Put a little drop of Canada

balsam, Arabian cement, or gum Arabic on the top of the cylinder; put

the object on it, press it into its place, and, when the cement is hard,

the preparation is complete. The cover of the box serves to keep the

object from dust.

Now we come to the Canada balsam, a substance which produces beautiful

effects when rightly handled, but is most aggravating to the learner,

causing alternate irascibility and depression of spirits. Many objects,

such as the antennæ and feet of insects, will not show their full beauty

unless they are mounted in Canada balsam. The method of doing so is as

follows:--A week or two beforehand put the objects into ether or spirits

of turpentine, and allow them to remain there until wanted. Pile up some

old books, or take a couple of convenient wooden blocks; lay your brass

plate upon them; light the spirit-lamp, and put it under the plate so as

to heat it. Lay two or three slides on the plate, and all then can be

heated at the same time.

Warm the bottle of Canada balsam, and with a glass rod take out a very

little drop, and put it exactly in the middle of the slide. In order to

insure this point, I always put a dot of ink on the wrong side of the

slide. Stir it about with one of the needles mentioned on page 428, and

if any bubbles rise, break them. When the balsam is quite soft and

liquid, take one of the objects out of the bottle and put it into the

balsam, exactly over the black dot. Now add a little more balsam, so as

to cover it, and let it lie for a few moments. Take one of the glass

covers, put a very little balsam on its centre, and lay it neatly over

the object, pressing it down gradually and equally. Unless this be done,

the object will not remain in the centre, but will shoot out on one

side, and the whole operation must be begun _de novo_. Remove it from

the hot plate and lay it on a cool surface, still continuing the

pressure until the balsam has begun to harden. Lay a little leaden

weight--a pistol-bullet partly flattened is excellent for the

purpose--and on the cover write the name of the object, as already

mentioned, and then proceed to prepare another slide.

Twenty such slides may be prepared in the course of a morning, and when

they are finished they should be laid carefully in a cold place, where

they will be free from dust. In a week or so the balsam will be quite

hard, and then the slide may be completed. Take an old knife, which

should be kept for this special purpose; heat the blade in the

spirit-lamp, and then run it along the edges of the slide, so as to take

off the superfluous balsam which has escaped from beneath the cover.

This must be done very quickly, or the balsam inside the cover will be

heated by the knife, and the preparation spoiled. When this is done, cut

the ornamental paper, as already described, number and label the slide,

wash off the ink, and then the preparation is complete. Some objects are

very troublesome to prepare, and require to be soaked in turpentine and

boiled repeatedly in the balsam before they are completely penetrated

with it.

Objects which are put up in Deane’s gelatine are managed after a similar

fashion, save that the gelatine is to be heated by placing the bottle in

hot water, and that the turpentine is not needed. Vegetable structures

show beautifully when thus prepared. To remove the superfluous gelatine

use a _wet_ and not a hot knife.

Cells are very difficult to manage, and the novice had better not

attempt to make them, but is hereby advised to purchase them ready made.

Suppose that the young microscopist has dissected the digestive organs

of a bee, and wishes to preserve it in spirit; his best plan will be to

use a cell for the purpose. Let him buy a cell of sufficient depth,

float the preparation into it, fill it up with spirit, put the cover

loosely on, and leave it for a week, occasionally raising the cover and

stirring the preparation with a needle, in order to get rid of any

air-bubbles that may have been entangled in the tissues.

Then let him wipe the edges of the cell very dry, put on a slight layer

of gold-size or asphalte varnish--the former is preferable--fill up the

cell a “bumper,” and lay the cover very gently upon it, beginning at one

end and gently lowering it. With blotting-paper the liquid that escapes

must be removed, the edges dried afresh, a flattened bullet placed on

the cover, and with a very small camel’s-hair brush the slightest

possible coating of size painted round the edge of the cell. When it has

hardened another may be given, and so on, until a thick hard wall of

size has been built up round the edges and made the cover completely

air-tight.

We presume that the reader does not intend to use his microscope merely

as a toy, but that he desires to gain some insight into the works of

Nature, and is therefore willing to set to work in a systematic manner.

It is now known that both animal and vegetable structures are built up

by means of certain minute particles, technically called CELLS, and that

in every part of a plant or of an animal can be recognised the

constituents of which it is formed. We will, therefore, begin with the

vegetables.

[Illustration: CELL, STRAWBERRY.]

Some of the lowest plants, such as the minute algæ that inhabit the

water, afford excellent examples of the simple vegetable cell; but as

these plants are not readily procured by a beginner, we will select some

familiar object wherein the cells may be found. If any soft and pulpy

fruit be taken when it is quite ripe, and submitted to the microscope,

the vegetable cell will be seen in a tolerably perfect form. The three

rounded objects shown in the accompanying illustration are cells from

the strawberry, specimens of which can easily be seen, if a very thin

slice be cut with a razor or lancet, the latter being the preferable

instrument. Be careful to dip the blade in water before cutting the

fruit, and to float the slice from the blade to the glass slide by

placing them both under water. Unless this precaution be taken, the

section will not be flat, but will be crumpled up, and the cells will

not be properly seen.

Within each of these cells may be seen a small rounded object, which is

technically called the “nucleus;” and in some cases a smaller nucleus,

called the “nucleolus,” may be observed within the nucleus itself. The

increase of cells mostly takes place by a process of division. A line

passes across the nucleus, which presently separates into two distinct

parts, each of which recedes from the other, causing the cell to enlarge

and alter its shape. Presently a line is seen across the cell itself,

and in due time the cell is also divided into two parts, each having its

own nucleus.

In the present instance the cell is totally spherical, because the fruit

from which it was taken was soft, and allowed the constituent cells to

expand. When, however, the vegetable substance becomes hard, the cells

are pressed closely together, and their shapes are very much altered.

Sometimes, when the cells are of nearly the same size, and the pressure

is equal on every side, the cells form regular twelve-sided figures,

called “dodecahedra,” which, when that occurs, show a six-sided outline.

A very thin slice of raw potato will show the twelve-sided cells

beautifully, and has the further advantage of exhibiting the starch

globules with which the cells are filled. Here is a figure of a potato

cell, which presents a six-sided outline, just like that of a bee’s

waxen dwelling, and which is crowded with the beautiful globules of

starch. If the reader likes to make a few dozen balls of clay, and to

squeeze them together in a mass, he will find that the central balls

will have lost their globular shape, and assumed a more or less regular

twelve-sided form, very much like that of the potato cells.

[Illustration: CELL, POTATO.]

[Illustration: STELLATE TISSUE.]

Sometimes the cells run out longitudinally into cylinders, and attain

the really enormous length of three inches; sometimes they become

flattened, as the skin or epidermis of many plants; and oftentimes they

push out their sides into arms or rays, like stars, and form the tissue

which is technically called “stellate.” Here is a specimen of stellate

tissue taken from the pith of the common rush, wherein the rays are seen

to be very regular: generally, however, the rays are extremely

irregular, and require some little practice to detect them. Stellate

tissue may be seen in the white portion of orange-peel, in the thick

fleshy substance of many aquatic plants, in certain leaf-stalks, and in

many similar objects.

We will now see how the soft cells which form the pulpy fruit of the

strawberry can be changed into the hard timber of the oak or iron-wood

tree.

[Illustration: RINGED STRUCTURE.]

Wherever a cell is destined to form part of a _permanent_ tissue, it is

strengthened by receiving certain additions to its walls. These

additions are technically known as “secondary deposit,” and are made in

various ways. Sometimes they extend in a thin layer over the whole

cell-wall, leaving a number of little holes, which are called “pits,”

and earning the name of “pitted structures.” Very frequently the

secondary deposit is arranged in a series of rings, an example of which

is given in the accompanying illustration. This object is taken from the

mistletoe. Good examples of the ringed structures may be seen in the

anthers of many plants, and in the leaf-stem of the common rhubarb, an

example of which is shown in the next illustration. Another very common

form of secondary deposit is the spiral, which is generally used where

strength and elasticity are united. Two examples of the spiral form are

given in the illustration; the first taken from the lily, and the second

from the “rhizome,” or subterranean stem of the water-lily.

[Illustration: RINGED AND SPIRAL STRUCTURES.]

Another beautiful form of secondary deposit is seen in the fern root. If

the root be cut longitudinally, and the dark hard fibre dissolved

carefully out with nitric acid, the deposit will seem to have assumed

the shape of a winding staircase, and is then called “scalariform,” or

ladder-shaped. Similar structures may be found in asparagus.

[Illustration: WOOD-CELLS.]

The reader will see that the hardness of the structure depends entirely

on the amount of secondary deposit, and we accordingly find that when

the wood is hard and fit to be worked with tools the cells are almost

wholly filled with the secondary deposit. In this state they are called

“wood-cells.” Examples of these cells may be seen in the accompanying

illustration. In the first example, which is taken from the elder-tree,

four cells are shown in order to display the manner in which their

pointed ends are arranged. (The reader must remember that in all

wood-cells the ends are pointed.) In the next example, which is taken

from the chrysanthemum, the pitted structure is still retained; but in

the last figure, which is drawn from the lime-tree, the entire cell is

filled with secondary structure. The reader must understand that we can

only give the veriest outline of the subject, and profess to do nothing

more than indicate the method of observation, leaving the pupil to work

out the details by himself.

[Illustration: HAIR OF LAVENDER.]

Another curious development of the plant-cells is the formation of

HAIRS. These objects alone afford an inexhaustible field for the

microscopist, and any one who chooses to work out the subject will find

himself repaid if he makes a good series of preparations. In their

primary forms the hairs are seen merely as little projections on the

epidermis, whether of the stem, leaf, or petal, and by degrees assume

their varied and beautiful forms. In order to show the singular forms

which hairs sometimes assume, an illustration is here given of the hairs

of the lavender leaf. This is one of the hairs that give the leaf its

silvery gloss. It consists of an upright stem, from the top of which a

number of forked branches shoot out horizontally, much like an open

umbrella held upright. The object of this remarkable form is, that the

delicate vessels in which the perfume is held should escape injury. If

the reader will refer to the second figure, which represents a much

magnified view of the edge of the leaf, he will see the globular

perfume-gland standing under the shelter of the branching hairs.

The following plants afford valuable examples of hair:--Arabis, marvel

of Peru, sowthistle, tobacco, southernwood, hollyhock, snapdragon, pansy

(in throat of flower), deutzia (under-side of leaf), verbena, alyssum,

tradescantia, borage, cowhage, and many others. The beautiful effect

produced by the petals of flowers is caused by the imperfect hairs with

which their surfaces are studded.

The POLLEN of plants is always worth observing, and some specimens are

of remarkably beautiful shapes. Take that of althæa, crocus, cactus,

heath, violet, daisy, lily, snowdrop, wallflower, willow-herb (a very

beautiful form), hollyhock, periwinkle, primrose, &c. Put some up in

Deane’s gelatine, and dry some, besides examining them all when fresh.

The microscopist ought to examine the structures of WOOD by making

sections in the directions transverse and longitudinal. A razor will

answer very well for the purpose, and the wood should always be soaked

inside, and the razor wetted before the section is made. It is often

useful to make diagonal sections of several woods, especially those of

the pine and juniper. All the forest trees should be examined, and their

roots and bark should not be omitted. Cut sections of coconut-shell,

vegetable ivory, sugar-cane (a most beautiful object when mounted

opaque), bamboo, butcher’s broom, &c.

MOSSES are beautiful objects, and can always be found. Examine

particularly the fruit or seed-vessel, and note the structure of its

different parts. Put these on a slide, and breathe on them, noting at

the same time any change which may take place.

The SPORE CASES of ferns are extremely beautiful, and should be

carefully examined. The little brown dots or streaks that are seen on

the under surface of the fronds are called “sori,” and contain a large

but variable number of the sporanges. These consist of stalked sacs or

cases, and differ much in shape, according to the species of fern. If

the fern be fresh from which the sorus is taken, the sporanges may be

seen writhing and twisting like so many serpents, and sometimes it

happens that one of the sporanges bursts, and suddenly covers the field

of the microscope with minute black dots. These dots are the spores or

seeds of the fern, and when magnified with a very high power, they are

seen to be variously shaped. One of the most remarkable spores is that

of the equisetum, or mare’s tail of the water. This spore looks like a

ball with something coiled round it. As soon as the spore is discharged

from its case, four threads are seen to uncoil themselves from around

it, and by their elasticity to cause the spore to jump about as if

alive. These fibres are technically named elasters, and are

prolongations of the outer coat of the spore.

FUNGI of all kinds should be examined. There is never any difficulty in

finding fungi, though the autumn is the best time of year for this

purpose. “Mould,” as it is popularly called, is a form assumed by many

species of fungus, which, though objectionable to the careful housewife,

are full of interest to the microscopist. The well-known mushroom and

toadstools are the highest of the fungi. The black spots on leaves are

fungi, mostly belonging to the genus puccinia, and the best specimens

are generally found on the wild rose or bramble. The black “smut” of

wheat is another fungus, very pretty under the microscope, but very

obnoxious to the farmer; and the “bunt” also belongs to the same vast

tribe of plants, four thousand species of which are now known to exist.

The young observer should also look for the beautiful crystals which

exist in many vegetable cells. The RAPHIDES, as these crystals are

called, are of various forms, mostly shaped like curved needles, but

often assuming very pretty and regular outlines. Raphides are

plentifully found in the bulb of the onion, in the rhubarb, the lily,

the iris, &c. They are best mounted as opaque objects and, if the reader

can procure a binocular microscope, he will see the form of the

raphides better than with the single-tube instrument.

SEEDS of different plants should be carefully examined, especially those

of small dimensions, which often exhibit some wonderful beauties of

structure. The winged seed of various plants, such as the thistle, the

dandelion, the valerian, and the willow-herb, are extremely interesting

objects; while those of the yellow snapdragon, the mullein, the Robin

Hood, and the bur-seed, are remarkably beautiful in form, though they

have no parachute, as the feathery appendage is called.

Leaving dry land, we will devote a short time to the water. Let the

reader take with him the simple collecting apparatus mentioned on page

430, and secure specimens of the water from different ponds, ditches,

and streams. For collecting the larger objects a little net, which can

be purchased cheap, is of very great use. It is easily made by any

tinman, and if the young microscopist knows the use of solder, as all

experimental philosophers ought to do, he can put it together in a few

minutes. It is formed of a strip of zinc bent into the requisite form,

and with a socket, to which a handle can be attached. A piece of coarse

muslin, or, rather, fine “net,” is then stretched over the bottom, and

the apparatus is complete.

In the water is sure to be found one of the lowest forms of vegetable

life--namely, the “confervoid algæ.” Look for these in bright, clear

pools, placing the collecting bottle near any greenish film collected

around the stems of plants, or spread over the stones on the bed of the

pool. If this film be very carefully taken up, it will produce many

interesting forms of vegetable life. One of the most remarkable of these

vegetables is that which is called “volvox globator,” a figure of which

is here given.

[Illustration: VOLVOX.]

This wonderful object is about as large as the head of a very small pin,

so that it is visible to the naked eye, and looks like a tiny globule

passing through the water. When it is placed under a lens of moderate

power, say of an inch focus, it exhibits some very strange

peculiarities. It continually revolves, and by its revolution is able to

enjoy a moderate degree of locomotion, though without any apparent

object. Small dark spots are also seen upon it.

If a half-inch lens be now used, the structure of the volvox begins to

be exhibited. The whole surface is covered with a network of very fine

fibres, having a spot at the intersection of each mesh. On applying a

still higher power, say the four-tenths of an inch, the structure is

further elucidated, and the dots on the surface are seen to consist of

greenish bodies, each furnished with a pair of delicate fibres,

technically named cilia, which are constantly vibrating, and cause the

revolution of the general mass. The dark spots are now seen to be the

young plants in different stages of progress. From six to ten of these

are inclosed within the parent, and when the latter has reached its full

age, the membrane bursts asunder, and the little volvoces are liberated.

[Illustration: CLOSTERIUM.]

Another interesting form is the closterium, a genus which is sure to

produce several good examples. We may mention that the ponds in

Blackheath are very rich in these curious vegetables, and a very

considerable series of confervoids may be obtained from them. The

closteria are easily recognised by their resemblance to the Australian

“boomerang.”

As our space is rapidly waning, we must leave the vegetable, and proceed

to the animal kingdom.

As is the case with vegetables, the animal structure is composed of

cells, though they cannot be so easily traced as in the examples which

we have already noticed. The young observer may readily perceive the

animal cell, in its largest and simplest form, by placing a little of

the yolk of egg under the microscope. CARTILAGE, or gristle, is easily

seen to be composed of cells. The nails of the fingers afford good

objects for the microscopist in search of animal cells. If a thin

section be placed under the microscope, none but an experienced observer

will be able to make out the presence of cells at all; but if the

section be soaked in “liquor potassæ,” the cells immediately swell up,

and their shape is at once made plain. Take the BONE of a young chicken

or rabbit, and make a thin section that embraces both the bone and

cartilage, and there will then be a beautiful object for the

microscopist, showing how the cartilage is changed by degrees into bone.

[Illustration: BONE, TRANSVERSE.]

Sections of bone should also be made, both transverse and longitudinal.

The BLOOD is another object which must be carefully examined. The

“corpuscles” which give the colouring matter to the blood are cells of

different size, according to the creature from which they are taken. The

dimensions of the animal exercise no apparent influence on the

corpuscles, for those of “proteus anguinus,” a little creature not

larger than a lamprey, are many times larger than those of the ox. In

the accompanying illustration is shown a series of specimens, in order

to show the great difference in their shape and size, all being drawn to

scale and magnified by the same lens. The circular corpuscles in the

left-hand upper corner are those of man; immediately below is a single

corpuscle from the pigeon. The great central corpuscle is taken from the

proteus; the two in the lower right-hand corner are from the frog, one

of these being viewed edgeways; and of the remaining two, that on the

left hand belongs to the tortoise, and that on the right to a fish.

[Illustration: BLOOD CORPUSCLES.]

The insect tribes are an inexhaustible source of objects for the

microscopist, who may find that even a single fly will give him

employment for many months. The scales from the butterfly’s wing, the

wonderful compound eyes with which insects are gifted, the structure of

their feet, and their entire anatomy, are always at the service of any

microscopist who really cares for his work. It would, of course, be

impossible to give even a list of the interesting portions of the

different insects; so one or two examples must suffice us.

[Illustration: ANTENNÆ OF FLY.]

Take the ANTENNÆ of the insect tribes, and see how beautifully they are

formed, how graceful is the shape, and how elaborate the structure. A

low power will be useful for exhibiting their general shape and outline,

but it is not until we know how to use the higher powers that the real

beauty of these curious organs is seen. In the accompanying illustration

is given part of an antenna of the common blue-bottle fly, in order to

show the remarkable cavities which exist within the antennæ, and which

are thought by some anatomists to be organs of hearing, and by others to

be organs of smell.

[Illustration: WINGS OF BEE.]

The WINGS of insects are also most remarkable, and possess many

peculiarities of structure which cannot be detected without the aid of a

microscope. Take, for example, the wings of any hymenopterous insect,

say those of a humble-bee, and see how beautiful is the structure which

causes the four wings to be united into two when the insect is about to

fly. In the illustration may be seen a pair of these wings, together

with the row of hooks which bind them together. A still more magnified

representation of the hooks is placed near the wings.

It is now ascertained that the wings of insects are connected with the

breathing apparatus, and that the respiration of the insect extends even

to the very tips of these singular organs, which are not modifications

of existing limbs, as in the birds, but additional structures. The

circulation of insects may often be seen by placing a portion of a

transparent wing under a moderately high power. We have often seen it in

the wing of the great water-beetle. A series of very beautiful

preparations may be made in order to show the distinction between the

wings of different insects; and as the orders of insects are founded

upon their wings, there ought to be at least one example of each order.

The proboscis of insects is always worthy of careful examination.

As to the breathing apparatus itself, the best mode of examining it is

to open a caterpillar, remove a part of the large breathing tube which

runs along each side, and place it under the microscope. It should

always be taken so as to include one of the spiracles, or

breathing-holes. An example of a breathing-tube, taken from a silk-worm,

is given in the illustration.

[Illustration: BREATHING-TUBE, SILKWORM.]

HAIRS of animals are very curious and interesting objects. They should

be mounted in three modes--namely, dry transparent, dry opaque, and in

Canada balsam, transparent. Be sure to procure some hair of the bat, the

sheep, the mouse, the deer, the mole, and any of the weasel tribe. Many

insects have very beautiful hair, but the most lovely hair in the animal

kingdom is that which is obtained from the sea-mouse. Fish scales should

also be procured, and specimens should be taken from the lateral line.

MOLLUSCS of all kinds afford many beautiful objects, and the observer

should be very careful to examine the wonderful tongue-ribbon of the

snail, the slug, the periwinkle, the whelk, and other similar molluscs.

If meant to be examined by polarized light, the tongue-ribbon should be

mounted in Canada balsam.

CRYSTALS should always form part of a collection. Take those of common

salt, nitre, sugar, chlorate of potash, salicine, &c.; indeed, anything

that will crystallize should be prepared and mounted, as such objects

will often be most useful when examining unknown substances.

[Illustration: BIRD’S-HEAD PROCESS.]

[Illustration: NOCTILUCA.]

[Illustration: PEDICILLARIÆ.]

ZOOPHYTES must of course find a place in the cabinet, and the young

microscopist ought to put up a few specimens of the “bird’s-head”

processes which are found in the bugularia and other inhabitants of the

sea. The pretty noctiluca, to which is mostly owing the phosphorescence

of the sea, should be preserved, and the extraordinary appendages to the

skin of certain star-fish and sea urchins should be examined. These are

called pedicillariæ, and a sketch of them is given in the illustration.

[Illustration]

OPTICS AND OPTICAL AMUSEMENTS.

“’Seeing is believing,’ so the sages say,

To prove this false, hear me, my friends, I pray,

And very soon you all will be agreeing,

That nought is so deceptive as our _seeing_.”--MARTIN.

Optics is the science of _light_ and _vision_. Concerning the nature of

light, two theories are at present very ably maintained by their

respective advocates. One is termed the Newtonian theory, and the other

the Huygenean. The Newtonian theory considers light to consist of

inconceivably small bodies emanating from the sun, or any other luminous

body. The Huygenean conceives it to consist in the undulations of a

highly elastic and subtle fluid, propagated round luminous centres in

spherical waves, like those arising in a placid lake when a stone is

dropped into the water.

LIGHT AS AN EFFECT.

Light follows the same laws as gravity, and its intensity or degree

decreases as the square of the distance from the luminous body

increases. Thus, at the distance of two yards from a candle we shall

have four times less light than we should have, were it only one yard

from it, and so on in the same proportion.

[Illustration]

REFRACTION.

Bodies which suffer the rays of light to pass through them, such as air,

water, or glass, are called refracting media. When rays of light enter

these, they do not proceed in straight lines, but are said to be

refracted, or bent out of their course, as seen in the drawing. The ray

of light proceeding from B through the glass L G is bent from the point

C, instead of passing in the direction of the dotted line. But if the

ray F C falls perpendicularly on the glass, there is no refraction, and

it proceeds in a direct line to K; hence refraction only takes place

when rays fall obliquely or aslant on the media.

[Illustration]

THE INVISIBLE COIN MADE VISIBLE.

If a coin be placed in a basin, so that on standing at a certain

distance it be just hid from the eye of an observer by the rim or edge

of the basin, and then water be poured in by a second person, the first

keeping his position; as the water rises the coin will become visible,

and will appear to have moved from the side to the middle of the basin.

[Illustration]

THE MULTIPLYING GLASS.

The multiplying glass is a semicircular piece of glass cut into facets

or distinct surfaces; and in looking through it we have an illustration

of the laws of refraction, for if a small object, such as a fly, be

placed at D, an eye at E will see as many flies as there are surfaces or

facets on the glass.

[Illustration]

TRANSPARENT BODIES.

Transparent bodies, such as glass, may be made of such form as to cause

all the rays which pass through them from any given point to meet in any

other given point beyond them, or which will disperse them from the

given point. These are called lenses, and have different names according

to their form. 1. Is called the plano-convex lens. 2. Plano-concave. 3.

Double convex. 4. Double concave. 5. A meniscus, so called from its

resembling the crescent moon.

[Illustration]

THE PRISM.

The prism is a triangular solid of glass, and by it the young optician

may decompose a ray of light into its primitive and supplementary

colours, for a ray of light is of a compound nature. By the prism the

ray A is divided into its three primitive colours, blue, red, and

yellow; and their four supplementary ones, violet, indigo, green, and

orange. The best way to perform this experiment is to cut a small slit

in a window-shutter, on which the sun shines at some period of the day,

and directly opposite the hole place a prism P; a beam of light in

passing through it will then be decomposed, and if let fall upon a sheet

of white paper, or against a white wall, the seven colours of the

rainbow will be observed.

[Illustration]

COMPOSITION OF LIGHT.

The beam of light passing through the prism is decomposed, and the

spaces occupied by the colours are in the following proportions:--red,

6; orange, 4; yellow, 7; green, 8; blue, 8; indigo, 6; violet, 11. Now,

if you paste a sheet of white paper on a circular piece of board about

six inches in diameter, and divide it with a pencil into fifty parts,

and paint colours in them in the proportions given above, painting them

dark in the centre parts, and gradually fainter at the edges, till they

blend with the one adjoining. If the board be then fixed to an axle, and

made to revolve quickly, the colours will no longer appear separate and

distinct, but becoming gradually less visible they will ultimately

appear _white_, giving this appearance to the whole surface of the

paper.

[Illustration]

A NATURAL CAMERA OBSCURA.

The human eye is a camera obscura, for on the back of it on the retina

every object in a landscape is beautifully depicted in miniature. This

may be proved by the

BULLOCK’S EYE EXPERIMENT.

Procure a fresh bullock’s eye from the butcher, and carefully thin the

outer coat of it behind: take care not to cut it, for if this should be

done the vitreous humour will escape, and the experiment cannot be

performed. Having so prepared the eye, if the pupil of it be directed to

any bright objects, they will appear distinctly delineated on the back

part precisely as objects appear in the instrument we are about to

describe. The effect will be heightened if the eye is viewed in a dark

room with a small hole in the shutter, but in every case the appearance

will be very striking.

THE CAMERA OBSCURA.

[Illustration]

This is a very pleasing and instructive optical apparatus, and may be

purchased for four or five shillings. But it may be easily made by the

young optician. Procure an oblong box, about two feet long, twelve

inches wide, and eight high. In one end of this a tube must be fitted

containing a lens, and be made to slide backwards and forwards so as to

suit the focus. Within the box should be a plain mirror reclining

backwards from the tube at an angle of forty-five degrees. At the top of

the box is a square of unpolished glass, upon which from beneath the

picture will be thrown, and may be seen by raising the lid A. To use the

camera place the tube with the lens on it opposite to the object, and

having adjusted the focus, the image will be thrown upon the

ground-glass as above stated, where it may be easily copied by a pencil

or in colours.

[Illustration]

The form of a camera obscura used in a public exhibition is as

follows:--D D is a large wooden box stained black in the inside, and

capable of containing from one to eight persons. A B is a sliding piece,

having a sloping mirror C, and a double convex lens F, which may with

the mirror C be slid up or down so as to accommodate the lens to near

and distant objects. When the rays proceeding from an object without

fall upon the mirror, they are reflected upon the lens F, and brought to

fall on the bottom of the box, or upon a table placed horizontally to

receive them, which may be seen by the spectator whose eye is at E.

THE CAMERA LUCIDA.

[Illustration]

This instrument consists of a glass prism, C, D, D, E, having four sides

covered. The sides C, D, being exposed to the object to be delineated,

rays pass through the glass and fall on the sloping side D, E; from this

they are reflected to the top, and finally pass out of the prism to the

eye;[11] now from the direction at which the rays enter the eye, it

receives them as if coming from an image at A, B, and if a sheet of

paper be placed below the instrument, a perfect delineation of the

object may be traced with a pencil. This is a very useful instrument to

young draughtsmen.

[11] The eye is to be applied to the little circular hole seen on the

upper surface.

THE MAGIC LANTERN.

[Illustration]

This is one of the most pleasing of all optical instruments, and it is

used to produce enlarged pictures of objects, which being painted on a

glass in various colours are thrown upon a screen or white sheet placed

against the wall of a large room. It consists of a sort of tin-box,

within which is a lamp, the light of which (strongly reflected by the

reflector T,) passes through a great plano-convex lens E fixed in the

front. This strongly illuminates the objects which are painted on the

slides or slips of glass, and placed before the lens in an inverted

position, and the rays passing through them and the lens F, fall on a

sheet, or other white surface, placed to receive the image. The glasses

on which the figures are drawn are inverted, in order that the images of

them may be erect.

PAINTING THE SLIDES.

The slides containing the objects usually shown in a magic lantern, are

to be bought at opticians with the lantern, and can be procured cheaper

and better in this way than by any attempt at manufacturing them.

Should, however, the young optician wish to make a few slides of objects

of particular interest to himself, he may proceed as follows:--

[Illustration]

Draw first on paper the figures you wish to paint, lay it on the table,

and cover it over with a piece of glass of the above shape; now draw the

outlines with a fine camel’s hair pencil in black paint mixed with

varnish, and when this is dry, fill up the other parts with the proper

colours, shading with bistre also mixed with varnish. The transparent

colours are alone to be used in this kind of painting.

TO EXHIBIT THE MAGIC LANTERN.

The room for the exhibition ought to be large, and of an oblong shape.

At one end of it suspend a large sheet so as to cover the whole of the

wall. The company being all seated, darken the room, and placing the

lantern with its tube in the direction of the sheet, introduce one of

the slides into the slit, taking care to invert the figures; then adjust

the focus of the glasses in the tube by drawing it in or out as

required, and a perfect representation of the object will appear.

EFFECTS OF THE MAGIC LANTERN.

Most extraordinary effects may be produced by means of the magic

lantern; one of the most effective of which is a

TEMPEST AT SEA.

This is effected by having two slides painted, one with the tempest as

approaching on one side, and continuing in intensity till it reaches the

other. Another slide has ships painted on it, and while the lantern is

in use, that containing the ships is dexterously drawn before the other,

and represents _ships in the storm_.

[Illustration]

The effects of sunrise, moonlight, starlight, &c., may be imitated, also

by means of double slides, and figures may be introduced sometimes of

_fearful_ proportions.

[Illustration]

Heads may be made to nod, faces to laugh; eyes may be made to roll,

teeth to gnash; crocodiles may be made to swallow tigers; combats may be

represented; but one of the most instructive uses of the slides is to

make them illustrative of astronomy, and to show the rotation of the

seasons, the cause of eclipses, the mountains in the moon, spots on the

sun, and the various motions of the planetary bodies, and their

satellites.

THE PHANTASMAGORIA.

Between the phantasmagoria and the magic lantern there is this

difference: in common magic lanterns the figures are painted on

transparent glass, consequently the image on the screen is a circle of

light having figures upon it; but in the phantasmagoria all the glass is

made opaque, except the figures, which, being painted in transparent

colours, the light shines through them, and no light can come upon the

screen except that which passes through the figure, as is here

represented.

[Illustration]

There is no sheet to receive the picture, but the representation is

thrown on a thin screen of silk or muslin placed between _the spectators

and the lantern_. The images are made to appear approaching and receding

by removing it further from the screen, or bringing it nearer to it.

This is a great advantage over the ordinary arrangements of the magic

lantern, and by it the most astonishing effects are often produced.

DISSOLVING VIEWS.

The dissolving views, by which one landscape or scene appears to pass

into the other while the scene is changing, are produced by using two

magic lanterns placed side by side, and that can be a little inclined

towards each other when necessary, so as to mix together the rays of

light proceeding from the lenses of each, which produces that confusion

of images, in which one view melts as it were into the other, which

gradually becomes clear and distinct; the principle being the gradual

extinction of one picture, and the production of another.

HOW TO RAISE A GHOST.

[Illustration]

The magic lantern, or phantasmagoria, may be used in a number of

marvellous ways, but in none more striking than in raising an apparent

spectre. Let an open box, A B, about three feet long, a foot and a half

broad, and two feet high, be prepared. At one end of this place a small

swing dressing-glass, and at the other let a magic lantern be fixed with

its lenses in a direction towards the glass. A glass should now be made

to slide up and down in the groove C D, to which a cord and pulley

should be attached, the end of the cord coming to the part of the box

marked A. On this glass the most hideous spectre that can be imagined

may be painted, but in a squat or contracted position, and when all is

done, the lid of the box must be prepared by raising a kind of gable at

the end of the box B, and in its lower part at E an oval hole should be

cut sufficiently large to suffer the rays of light reflected from the

glass to pass through them. On the top of the box at F place a

chafing-dish, upon which put some burning charcoal. Now light the lamp G

in the lantern, sprinkle some powdered camphor or white incense on the

charcoal, adjust the slide on which the spectre is painted, and the

image will be thrown upon the smoke. In performing this feat the room

must be darkened, and the box should be placed on a high table, that the

hole through which the light comes may not be noticed.

THE THAUMATROPE.

This word is derived from two Greek words, one of which signifies

_wonder_, and the other _to turn_. It is a very pretty philosophical

toy, and is founded upon the principle in optics, that an impression

made upon the retina of the eye lasts for a short interval after the

object which produced it has been withdrawn. The impression which the

mind receives lasts for about the eighth part of a second, as may be

easily shown by whirling round a lighted stick, which if made to

complete the circle within that period, will exhibit not a fiery point,

but a fiery circle in the air.

THE BIRD IN THE CAGE.

Cut a piece of cardboard of the size of a penny piece, and paint on one

side a bird, and on the other a cage; fasten two pieces of thread one on

each side at opposite points of the card, so that the card can be made

to revolve by twirling the threads with the finger and thumb: while the

toy is in its revolution, the bird will be seen within the cage. A bat

may in the same manner be painted on one side of the card, and a

cricketer upon the other, which will exhibit the same phenomenon,

arising from the same principle.

[Illustration]

CONSTRUCTION OF THE PHANTASMASCOPE.

The above-named figure is a Thaumatrope, as much as the one we are about

to describe, although the term Phantasmascope is generally applied to

the latter instrument; which consists of a disc of darkened tin-plate,

with a slit or narrow opening in it, about two inches in length. It is

fixed upon a stand, and the slit placed upwards, so that it may easily

be looked through. Another disc of pasteboard, about a foot in diameter,

is now prepared and fixed on a similar stand, but with this difference,

that it is made to revolve round an axis in the centre. On this

pasteboard disc, paint in colours a number of frogs in relative and

progressive positions of leaping; make between each figure a slit of

about a quarter of an inch deep: and when this second disc is made to

revolve at a foot distance behind the first, and the eye is placed near

the slit, the whole of the figures, instead of appearing to revolve with

the disc, will all appear in the attitudes of leaping up and down,

increasing in agility as the velocity of the motion is increased. It is

necessary, when trying the effect of this instrument, to stand before a

looking-glass, and to present the painted face of the machine towards

the glass.

A very great number of figures may be prepared to produce similar

effects--horses with riders in various attitudes of leaping, toads

crawling, snakes twisting and writhing, faces laughing and crying, men

dancing, jugglers throwing up balls, &c.; all of which, by the peculiar

arrangement above detailed, will seem to be in motion. A little

ingenuity displayed in the construction and painting of the figures upon

the pasteboard disc will afford a great fund of amusement.

CURIOUS OPTICAL ILLUSIONS.

One of the most curious facts relating to the science of vision is the

absolute insensibility of a certain portion of the retina to the

impression of light, so that the image of any object falling on that

point would be invisible. When we look with the right eye, this point

will be about fifteen degrees to the right of the object observed, or to

the right of the axis of the eye, or the point of most distinct vision.

When looking with the left eye, the point will be as far to the left.

The point in question is the basis of the optic nerve, and its

insensibility to light was first observed by the French philosopher,

Mariotte. This remarkable phenomenon may be experimentally proved in the

following manner:--

Place on a sheet of writing-paper, at the distance of about three inches

apart, two coloured wafers; then, on looking at the left-hand wafer with

the right eye, at the distance of about a foot, keeping the eye straight

above the wafer, and both eyes parallel with the line which forms the

wafers, the left eye being closed, the right-hand wafer will become

invisible; and a similar effect will take place if we close the right

eye, and look with the left.

ANOTHER.

Cut a circular piece of white paper, about two inches in diameter, and

affix it to a dark wall. At the distance of two feet on each side, but a

little lower, make two marks; then place yourself directly opposite the

paper, and hold the end of your finger before your face, so that when

the right eye is open it shall conceal the mark on your left, and when

the left eye is open the mark on your right. If you then look with both

eyes at the end of your finger, the paper disc will be invisible.

ANOTHER.

Fix a similar disc of paper, two inches in diameter, at the height of

your eye on a dark wall; a little lower than this, at the distance of

two feet on the right hand, fix another of about three inches in

diameter; now place yourself opposite the first sheet of paper, and,

shutting the left eye, keep the right eye still fixed on the first

object, and when at the distance of about ten feet, the second piece of

paper will be invisible.

THE PICTURE IN THE AIR.

One of the numerous optical illusions which have from time to time been

evolved by scientific minds, is that of making an image or picture

appear in the air. This is produced by means of a mirror, and an object

in relief, upon which a strong light is thrown,--the mirror being set at

such an angle as to throw up the reflection of the image to a certain

point in the view of the spectator. This illusion is produced as

follows: Let a screen be constructed in which is an arched aperture, the

centre of which may be five feet from the floor: behind the screen is

placed a large mirror of an elliptical form. An object is now placed

behind the screen, upon which the light of a strong lamp is thrown from

a point above the mirror, and is received by the mirror and reflected to

the centre of the arched cavity in the screen, where it will appear to

the spectator. Care should be taken to place the image in an inverted

position, and the light, which must be very powerful, should be so

placed that none of it may reach the opening.

BREATHING LIGHT AND DARKNESS.

[Illustration]

The following experiment, if performed with care, is exceedingly

striking. Let S be a candle, whose light falls at an angle of 56° 45′

upon two plate glasses, A B, placed close to each other; and let the

reflected rays, A C, B D, fall at the same angle upon two similar

plates, C D, but so placed that the plane of reflection from the latter

is at right angles to the plane of reflection from the former. An eye

placed at E, and looking at the same time on the two plates, C and D,

will see very faint images of the candle, S; which by a slight

adjustment of the plates, may be made to disappear almost wholly,

allowing the plate C to remain where it is. Change the position of D,

till its inclination to the ray, B D, is diminished about 3°, or made

nearly 53° 11′. The distance may be easily found by a little practice.

When this is done, the image that had disappeared on looking into D will

be restored, so that the spectator at E, upon looking into the two

mirrors, C D, will see no light in C, because the candle has nearly

disappeared, while the candle is distinctly seen in D. If, while, the

spectator is looking into these two mirrors, either he or another person

breathes upon them gently and quickly, the breath will revive the

extinguished image in C, and will extinguish the visible image in D.[12]

[12] Explanation.--The light A, C, B, D is polarized by reflection

from the plates A B, because it is incident at the polarizing

angle 56° 45’ for glass. When we breathe upon the plates C D, we

form upon their surface a thin film of water, whose polarizing

angle is 53° 11′, so that if the polarized rays A, C, B, D fall

upon the plates C, D, at an angle of 53° 11′, the candle from

which they proceeded would not be visible, or they would not

suffer reflection from the plates C D. At all the other angles

the light would be reflected, and the candle visible. Now the

plate D is placed at an angle of 53° 11′, and C at an angle of

56° 45′, so that when a film of water is breathed upon them, the

light will be reflected from the latter, and none from the

former; that is, the act of breathing upon the glass plates will

restore the invisible, and extinguish the visible image.

TO SHOW THAT RAYS OF LIGHT DO NOT OBSTRUCT EACH OTHER.

[Illustration]

Make a small hole in a sheet of pasteboard, A, and placing it upright

before three candles, B, placed closely together, it will be found that

the images of all the candle flames will be formed separately on a piece

of paper, C, laid on the table to receive them. This proves that the

rays of light do not obstruct each other in their progress, although all

cross in passing through the hole.

OPTICS OF A SOAP-BUBBLE.

[Illustration]

If a soap-bubble be blown up, and set under a glass, so that the motion

of air may not affect it, as the water glides down the sides and the top

grows thinner, several colours will successively appear at the top, and

spread themselves from thence down the sides of the bubble, till they

vanish in the same order in which they appeared. At length, a black spot

appears at the top, and spreads till the bubble bursts.[13]

[13] The thinnest substance ever observed is the aqueous film of the

soap-bubble previous to bursting; yet it is capable of reflecting

a faint image of a candle, or the sun. Hence its thickness must

correspond with what Sir Isaac Newton calls the beginning of

black, which appears in water at the thickness of the seven

hundred and fifty thousandth part of an inch.

THE KALEIDOSCOPE.

[Illustration]

[Illustration]

If any object be placed between two plane mirrors, inclined towards each

other at an angle of thirty degrees, three several images will be

perceived in the circumference of a circle. On this principle is formed

the kaleidoscope, invented by Sir David Brewster, and by means of which

the reflected images viewed from a particular point exhibit symmetrical

figures, under an infinite arrangement of beautiful forms and colours.

The kaleidoscope may be bought at any toy-shop, but it is requisite that

every young person should be able to construct one for himself. He must,

therefore, procure a tube of tin or paper, of about ten inches in

length, and two and a half or three inches in diameter. One end of this

should be stopped up with tin or paper, securely fastened, in which is

to be made a hole, about the size of a small pea, for the eye to look

through. Two pieces of well-silvered looking-glass, B B, are now to be

procured; they must be not quite so long as the tube, and they should be

placed in it lengthways, at an angle of 60 degrees, meeting together in

a point at A, and separating to the points C C, the polished surfaces

looking inwards. A circular piece of the glass is now to be laid on the

top of the edges of the reflectors, B B; which, by their not being quite

so long as the tube, will allow room for its falling in, and it will be

supported by the edges of the tube, which may be slightly bent over, to

prevent the glass from falling out. This having been done, now proceed

to make the “cap” of the instrument. A rim of tin or pasteboard must be

cut, so as to fit over the glass end of the tube; and in this, on the

outer side, a piece of ground glass must be fastened, so that the whole

may fit on the tube like the lid of a pill-box. Then, before putting it

on, obtain some small pieces of broken glass of various colours, beads,

little strips of wire, or any other object, and place them in the cap;

and by passing it over the end, so that the broken glass, &c. has free

motion, the instrument is complete. To use it, apply the eye to the

small hole, and, on turning it, the most beautiful forms will appear, in

the most wonderful combinations.

The following curious calculation has been made of the number of changes

this instrument will admit of. Supposing it to contain 20 small pieces

of glass, and that you make 10 changes in a minute, it will take an

inconceivable space of time, _i. e._ 462,880,899,576 years, and 360

days, to go through the immense number of changes of which it is

capable.

SIMPLE SOLAR MICROSCOPE.

Having made a circular hole in a window-shutter, about three inches in

diameter, place in it a glass lens of about twelve inches focal

distance. To the inside of the hole adapt a tube, having at a small

distance from the lens a slit, capable of receiving one or two very thin

plates of glass, to which the object to be viewed must be affixed by

means of a little gum-water exceedingly transparent. Into this tube fit

another, furnished at its extremity with a lens half-an-inch focal

distance. Place a mirror before the hole of the window-shutter on the

outside, in such a manner as to throw the light of the sun into the

tube, and you will have a solar magic lanthorn.

The method of employing this arrangement of lenses for microscopic

purposes is as follows:--Having darkened the room, and by means of the

mirror reflected the sun’s rays on the glasses in a direction parallel

to the axis, place some small object between the two moveable plates of

glass, or affix it to one of them with very transparent gum-water, and

bring it exactly into the axis of the tube; if the moveable tube be then

pushed out or drawn in, till the object be a little beyond the focus, it

will be seen painted very distinctly on a card, or piece of white paper,

held at a proper distance, and will appear to be greatly magnified. A

small insect will appear as a large animal, a hair as big as a

walking-stick, and the almost invisible eels in paste or vinegar as

large as common eels.

ANAMORPHOSES.

This is a very curious optical effect, producing a distorted and

grotesque figure from a regular one. The term is derived from two Greek

words, signifying a distortion of figure, and by its means many optical

puzzles may be produced geometrically.

Take any subject, such as the portrait of a head; divide it vertically

and horizontally with parallel lines, of which the outer sides shall

form the boundary, A, B, C, D, and the whole shall be equidistant. Then,

on a separate piece of paper, or cardboard, prepare a drawing similar to

Fig. 2 by the following means:--

1. Draw a horizontal line, _a b_, equal to A B, and divide it into as

many equal parts as the latter is divided.

2. Let fall a perpendicular line, _e v_, from the middle of _a b_, and

then draw _s v_ parallel to _a b_.

3. Both _e v_ and _s v_ may be any length at pleasure, but the longer

the first is, and the shorter the other, so will the anamorphoses be

more and more deformed. The proportions in our figures are sufficiently

different.

4. After having drawn from the point _v_ right lines, _v 1_, _v 2_, _v

3_, _v 4_, to the divisions of _a b_, draw the line _s b_, and through

each point where _s b_ intersects the divergent lines draw other

horizontal lines parallel to _a b_. We now have a _trapezium_, _a b c d_

divided into as many cells as the square in Fig. 1.

[Illustration: _Fig. 1._]

[Illustration: _Fig. 2._]

The next step is to fill up all the cells of Fig. 2 with portions of the

device, proportionate to their position in Fig. 1. For instance, in Fig.

1 the nose is in the second vertical division from the left, and in the

third and fourth horizontal divisions from the top, and that portion of

the face must accordingly be placed in a corresponding part of Fig. 2.

By these means we procure the anamorphosis seen in Fig. 2, which when

viewed from a particular position, will lose all its distortion, and

assume an appearance resembling that in Fig. 1. This position lies

immediately over the point _v_, and at a height above it equal to the

length of the line _s v_; and the means of determining it are as

follows.

Place the drawing horizontally before a window; take a slip of card, and

rest its lower edge on the line _s v_, the card being accurately

vertical; pierce a small hole in the card vertically over the point _v_,

and at a height from it equal to the length of the line _s v_, then with

the eye placed immediately behind the card, look through the orifice at

the anamorphosis, and it will be found that as soon as it has become

accustomed to the novelty of the experiment, the anamorphosis will lose

its distortion, and appear almost exactly like the symmetrical figure.

It would be very difficult, and would require geometrical reasoning of a

lengthened kind, to show why this particular form of construction should

lead to such results.

THE COSMORAMA.

[Illustration]

The principle upon which the cosmorama is formed is so simple, that any

person may easily fit up one in a small summer house, &c. Nothing more

is necessary than to fix in a hole a double convex lens of about three

feet focus, A, and at rather less than this distance a picture, B, is to

be hung. To absorb all the rays of light but those necessary for seeing

the picture, a squared frame of wood blackened on the inside is placed

between the lens and the picture. The picture may be hung in a large box

having a light coming in upon it from above, or in a small closet

illuminated in the same manner. Should it be wished to show it by

candle-light, a lamp, _c_, may be placed on the top of the wooden frame,

and if the light of this be converged by a lens to a moderate radius, it

will be more effective.

DISTORTED LANDSCAPES.

[Illustration]

Landscapes or other matters may be drawn so as to produce curious

optical illusions by the following method. Take a piece of smooth white

pasteboard and sketch the design upon it. Prick the outlines in every

part with a fine pin or needle, then place the pricked drawing in a

perpendicular position, and put a lighted candle behind it. Place before

it another piece of pasteboard, and follow with a pencil the lines given

by the light, and you have produced a distorted landscape. Now take away

the candle and the pricked drawing, and place your eye where the light

was, and the drawing will assume the regular form. To get your eye in

the proper position, it will be advisable to cut out a piece of card

according to the accompanying pattern, and raising it on its base, B,

look through the hole at A, when the object will appear in its proper

proportions.

[Illustration]

[Illustration]

PHOTOGRAPHY.

“Come like shadows, but not so depart.”--SHAKSPEARE.

Associated with the use of iodine and bromine is an art which every

intelligent boy may practise, if he will attend to the following precise

details kindly furnished by Mr. Richard Thomas, of 10, Pall Mall, who

has devoted many years to the careful study of all the processes

connected with it, and from whom all the best apparatus and purest

chemicals required may be obtained.

HOW TO MAKE THE NEGATIVE ON GLASS, USING COLLODION BROMO-IODIZED FOR

IRON DEVELOPMENT.

1. The edges of the glass should be ground all round, also slightly on

the surface of the edges. This prevents contraction of the film,

enabling it to resist the action of a heavy stream of water. Mark one

side in the corner with a diamond, and upon this side bestow the

greatest care.

2. _To clean the glass, if new._--Make a mixture of spirits of wine and

solution of ammonia, equal parts; render it as thick as cream with

tripoli; with a piece of cotton-wool kept for this purpose rub a small

quantity over that side marked as described, wash well under a tap of

water, and wipe dry with a piece of old linen, washed without soap, and

kept scrupulously clean for this purpose. Plates should not, however, be

cleaned in the operating room with the above mixture; the vapour of

ammonia might prove injurious to the chemicals.

3. Now polish with an old white silk handkerchief. If this latter

precaution be not taken, small particles of linen will be left upon the

plate: these are perhaps only seen when draining off the collodion; they

form nuclei and eddies, checking the collodion in its course. Some of

these minute fibres are washed off, and contaminate the next picture. To

all lovers of clean pictures our advice therefore is, having well dried

the plate with old linen, lay it, clean side upwards, upon a few sheets

of common glazed demy paper (not blotting), and rub it hard with the

silk until sensibly warm: this has the double advantage of dispersing

fibres and moisture, for all glass plates are slightly in a hygrometric

condition. Double the silk rubber up to form a pad, and with this the

glass must be firmly dusted down just before pouring on the collodion,

which will then run most evenly: if the coated plate is now viewed by

transmitted light, not a speck or blemish will be seen upon it. When a

plate cleaned as above described is breathed upon, the moisture does not

evaporate slowly, but _flies off_. Do not be afraid of putting the glass

into an electrical condition with the silk rubber: on this account

objections have been raised to the use of silk: practically, however, I

find it a most valuable auxiliary in this starting-point of the process,

the perfect manipulation of which makes an important difference in the

value of the finished picture. What can be more inartistic and annoying

to an educated eye than spots, patches, stars, and sky rockets, the

forms and shapes of which rival, in numberless variety, a display of

fireworks? Let us not, therefore, be contented with pictures, however

good in other respects, presenting these deformities--so many blots on

the photographic escutcheon.

_To clean a glass after having used it, when not varnished._--Wash off

the collodion film with water, then clean the marked side with plain

tripoli and water, and dry as above.

_To coat the plate._--First remove all the particles of dried collodion

from the mouth of the bottle. Now pour upon the centre of the cleaned

glass as much collodion as it will hold. Do not perform this operation

hurriedly, take time, and systematically incline the plate in such a

manner that the collodion may run into each corner in succession; when

perfectly covered, pour off gently the excess into the bottle at one of

the corners nearest to you: with observation and practice dexterity is

easily acquired. There are many ways of coating the plate; each person

will adopt that which practice teaches him is best. The pneumatic

plate-holder is a convenient little instrument to use for holding the

plate whilst pouring on the collodion: it may be used for both small and

large plates.

Keep the corner of the glass plate in contact with the neck of the

bottle whilst pouring off the collodion; otherwise the film will be wavy

in places.

4. As soon as the collodion ceases to run, plunge the prepared glass

gently, without stopping, into the nitrate of silver bath, which is

prepared as follows. Into a 20-oz. stoppered bottle put nitrate of

silver, 1¼ ozs.; distilled water, 4 ozs.: dissolve. To this solution add

iodide of potassium, 4 grs., dissolved in 1 drachm of distilled water.

Mix these two solutions: the precipitate (iodide of silver) thus formed

is by shaking entirely dissolved. Add 16 ozs. of distilled water, when

the excess of iodide of silver is again thrown down, but in such a

finely divided state as to render the saturation of the bath with iodide

of silver perfect. Now drop in sufficient of the oxide of silver to turn

the turbid yellow solution a dirty brown colour; so long as this effect

is produced the quantity of oxide of silver, however much in excess, is

of no consequence; shake the bottle well for ten minutes or so at

intervals; then add alcohol, 30 minims, and filter; to the filtered

solution add dilute nitric acid of the strength stated, 5 minims. The

bath is now ready for use, and should be quite neutral.

5. Allow the prepared glass to remain in this bath from five to ten

minutes, according to the temperature. Move it up and down three or four

times whilst in the bath, in order to get rid of the greasy appearance

on the surface: drain it, but not too closely. When in the frame, place

upon the back a piece of common blotting-paper, to absorb moisture, and

the two lower silver wires should also be covered with slips of

blotting-paper; after which the sooner it is placed in the camera the

better.

6. The time of exposure can only be ascertained by practice--no rules

can be laid down; and I am unacquainted with any royal road, but that of

experience, leading to constant success in this most important point.

7. The plate having been taken from the camera, and placed upon a

levelled stand, or held in the hand, develop immediately the latent

image with the following solution:--

_Iron developing solution._--Protosulphate of iron, ¼ oz.; glacial

acetic acid, ¼ oz.; spirits of wine, ½ oz.; distilled water, 8 ozs.:

mix. Pour on of this solution only enough to cover the plate easily,

commencing at that edge of the negative which stood uppermost in the

camera; move the solution to and fro until it has become intimately

mixed with the silver on the plate; then pour off into the developing

glass, and _at once_ return it on to the plate. When as much intensity

has been obtained as possible with the iron developer, it should be

thoroughly removed by washing with water. Any intensity may be obtained

afterwards by using either of the following solutions:--

8. _Intensifying solution._--Pyrogallic acid, 6 grs.; glacial acetic

acid, ¼ oz.; distilled water, 6 ozs.: mix. A few drops of a 30-gr.

solution of nitrate of silver, the quantity to be regulated according to

the intensity required, to be added, at the moment of using, to as much

of the pyrogallic solution as may be necessary.

_Intensifying solution_ (another form).--1. Pyrogallic acid, 8 grs.;

citric acid, 20 grs.; distilled water, 2 ozs. 2. Nitrate of silver, 8

grs.; distilled water, 2 ozs. Mix small quantities of the solutions 1

and 2, in equal portions, the moment before using.

The pyrogallic solution, made with good acetic acid, may be kept for a

month or more in a cool place. Nevertheless, if the conditions of light

and situation are unfavourable, I should prefer this solution just made.

The iron solutions act best when freshly prepared.

It is supposed by some that a prolonged action of the iron developer

produces fogginess. This may be the case when impure or improperly

prepared collodion is used, but certainly not when the preparation is

pure and of the proper quality.

When the image is sufficiently intense, wash freely with common filtered

water; then pour on a saturated solution of hyposulphate of soda, which

should immediately remove the iodide of silver: wash again well with

water; allow as much as the plate will hold to soak in for at least a

quarter of an hour, changing the water occasionally, to remove all

traces of hyposulphate; lastly, wash the plate with a little distilled

water, stand up to dry, and, if required, varnish either with spirit or

amber varnish.

The following solution is also very commonly used for fixing the

negative:--Cyanide of potassium, ¼ oz.; water, 12 ozs.

Attention to the following rules and cautions will assist the operator

in the production of perfect pictures:--

1. Do not disturb the deposit which will occasionally be found at the

bottom of the bottle containing the collodion.

2. Remove all particles of dried film from the neck of the bottle before

pouring the collodion on the plate.

3. Never use damp cloths, leathers, or buffs, for giving the final

polish to the plate. Negatives with an indistinct and muddy surface are

frequently produced from this cause.

4. Let the film set properly before immersion in the nitrate of silver

bath: its condition can be ascertained by gently touching the lower part

of the coated plate with the end of the finger.

5. Never omit to pass a broad camel-hair brush over the plate just

before pouring on the collodion.

6. Bear in mind that, as light is the producing agent, so will it prove

a destructive one: not less than four folds of yellow calico should be

used to obstruct white light; and in that case the aperture covered

should be no larger than is necessary to admit sufficient light for

working by. Examine occasionally the yellow calico: when this material

is used to exclude white light, it becomes bleached by constant

exposure. Do not trust alone to any coloured glass; no glass yet made is

anti-actinic under all aspects of light and conditions of exposure.

7. When the negative requires intensifying, carefully wash off all

traces of the first developing solution before proceeding to intensify.

This operation may be performed either before or after the iodide is

removed by fixing.

8. Glass baths are preferable to porcelain, ebonite, or gutta-percha

baths for solution of nitrate of silver.

9. In using either spirit or amber varnish, before pouring it off, keep

the plate horizontal a few seconds. This gives time for soaking in, and

prevents the formation of a dull surface arising from too thin a

coating.

10. Rub the lenses occasionally with a soft and clean wash-leather; the

rapidity of action is much influenced by the brightness of the lenses:

their surfaces are constantly affected by moisture in the atmosphere,

which condensing, destroys the brilliancy of the image.

11. The white blotting-paper used for some photographic purposes is not

suitable for filtering solutions; that only should be employed which is

made for this purpose, and is sold under the name of filtering-paper.

12. _Hyposulphate of soda._--A great deal of rubbish is sold under the

name of this salt. As a test of its quality, 1½ drachms should entirely

dissolve in 1 drachm of water, and this solution should dissolve rather

more than 4½ grains of iodide of silver.

13. _Chemicals._--The purity of photographic chemicals cannot be too

strongly urged; the cheapest are not always the most economical. The

commercial preparations are generally not to be depended upon, as these,

though perhaps unadulterated, are, strictly speaking, not chemically

pure. It is best to procure them from well-known chemists, who

understand the purpose for which they are intended, and make the

preparation of these substances peculiarly a branch of their business.

14. Never leave chemical solutions exposed in dishes: when done with,

pour them back into glass-stoppered bottles, and decant for use from any

deposit, or filter if necessary.

15. In all photographic processes it is absolutely necessary to be

chemically clean; and this sometimes is not easy. As a rule, never be

satisfied with cleanly appearances only, but take such measures as shall

insure the absence of all extraneous matter in preparing the solutions,

cleaning the glasses, dishes, &c.

16. All stains on the hands, linen, &c. may be removed by means of

cyanogen soap or cyanide of potassium, which should be applied without

water at first, then thoroughly washed off. To assist the operation, the

hands may be now gently rubbed with a fine piece of pumice-stone, when

the stains quickly disappear.

For more perfect and complete directions the reader is referred to a

most excellent work, “The Modern Practice of Photography,” by R. W.

Thomas, F.C.S.

[Illustration]

PNEUMATICS.

“There is a tricksy spirit in the air

That plays sad gambols.”--BEN JONSON.

The branch of the physical sciences which relates to the air and its

various phenomena is called Pneumatics. By it we learn many curious

particulars. By it we find that the air has weight and pressure, colour,

density, elasticity, compressibility, and some other properties with

which we shall endeavour to make the young reader acquainted by many

pleasing experiments, earnestly impressing upon him to lose no

opportunity of making physical science his study.

The common leather sucker by which boys raise stones will show the

pressure of the atmosphere. It consists of a piece of soft but firm

leather, having a piece of string drawn through its centre. The leather

is made quite wet and pliable, and then its under part is placed on the

stone and stamped down by the foot. This pressing excludes the air from

between the leather and the stone, and by pulling the string a vacuum is

left underneath its centre; consequently the leather is firmly attached

to the stone, which enables you to lift it.

WEIGHT OF THE AIR PROVED BY A PAIR OF BELLOWS.

Shut the nozzle and valve-hole of a pair of bellows, and after having

squeezed the air out of them, if they are perfectly air-tight, we shall

find that a very great force, even some hundreds of pounds, is necessary

for separating the boards. They are kept together by the weight of the

air which surrounds them in the same manner as if they were surrounded

by water.

THE PRESSURE OF THE AIR SHOWN BY A WINE-GLASS.

Place a card on a wine-glass filled with water, then invert the glass:

the water will not escape, the pressure of the atmosphere on the outside

of the card being sufficient to support the water.

[Illustration]

ANOTHER.

[Illustration]

Invert a tall glass jar in a dish of water, and place a lighted taper

under it; as the taper consumes the air in the jar, the water, from the

pressure without, _rises up_ to supply the place of the oxygen removed

by the combustion. In the operation of cupping the operator holds the

flame of a lamp under a bell-shaped glass. The air within this being

rarefied and expanded, a considerable portion is given off. In this

state the glass is placed upon the flesh, and as the air within it cools

it contracts, and the glass adheres to the flesh by the difference of

the pressure of the internal and external air.

ELASTICITY OF THE AIR.

[Illustration]

This can be shown by a beautiful philosophical toy, which may easily be

constructed. Procure a glass jar such as is here represented, and put

water into it. Then mould three or four little figures in wax, and make

them hollow within, and having each a minute opening at the heel, by

which water may pass in and out. Place them in the jar, as seen in the

figure, and adjust them by the quantity of water admitted to them, so

that in specific gravity they differ a little from each other. The mouth

of the jar should now be covered with a piece of skin or india-rubber,

and then, if the hand be pressed upon the top or mouth of the jar, the

figures will be seen to rise or descend as the pressure is gentle or

heavy; rising and falling, or standing still, according to the pressure

made.

REASON FOR THIS.

The reason of this is, that the pressure on the top of the jar condenses

the air between the cover and the water surface; this condensation then

presses on the water below, and influences it through its whole extent,

compressing also the air in the figures, forcing as much more water into

them as to render them heavier than water, and therefore heavy enough to

sink.

THE AIR-PUMP.

The time was, and that not very long ago, when the air-pump was only

obtainable by the philosophical professor or by persons of enlarged

means. But now, owing to our “cheap way of doing things,” a small

air-pump may be obtained for about a guinea, and we would strongly

advise our young friends to procure one, as it will be a source of

endless amusement to them; and, supposing that they take our advice, we

suggest the following experiments.

[Illustration]

The air-pump consists of a bell glass, called the receiver, A, and a

stand, upon which is a perforated plate, B. The hole in this plate is

connected with two pistons, the rods of which are moved by a wheel

handle backwards and forwards, and thus pump the air out of the

receiver. When the air is thus taken out, a stop-cock is turned, and

then the experiments may be performed.

Under the receiver of an air-pump, when the air has been thoroughly

exhausted, light and heavy bodies fall with the same swiftness. Animals

quickly die for want of air, combustion ceases, a bell sounds faint, and

water and other fluids change to vapour.

TO PROVE THAT AIR HAS WEIGHT.

Take a florence flask, fitted up with a screw and fine oiled silk valve.

Screw the flask on the plate of the air-pump, exhaust the air, take it

off the plate, and weigh it. Then let in the air, and again weigh the

whole, and it will be found to have increased by several grains.

[Illustration]

TO PROVE AIR ELASTIC.

Place a bladder out of which all the air has apparently been squeezed

under the receiver, upon it lay a weight, exhaust the air, and it will

be seen that the small quantity of air left within the bladder will so

expand itself as to lift the weight. Put a corked bottle into the

receiver, exhaust the air, and the cork will fly out.

SOVEREIGN AND FEATHER.

[Illustration]

Place a nicely-adjusted pair of forceps at the top of the receiver,

communicating with the top of the outside through a hole, so that they

may be opened by the fingers. Then place on each of the little plates a

_sovereign_ and a _feather_. Exhaust the air from the receiver; and

having done so, detach the objects, so that they may fall. In the open

air the sovereign will fall long before the feather, but in vacuo, as in

the receiver now exhausted of its air, they will fall both together, and

reach the bottom of the glass at the same instant.

AIR IN THE EGG.

Take a fresh egg, and cut off a little of the shell and film from its

smaller end; then put the egg under a receiver, and pump out the air;

upon which all the contents of the egg will be forced out by the

expansion of the small bubble of air contained in the great end between

the shell and the film.

THE DESCENDING SMOKE.

Set a lighted candle on the plate, and cover it with a tall receiver.

The candle will continue to burn while the air remains, but when

exhausted will go out, and the smoke from the wick, instead of rising,

will descend in dense clouds towards the bottom of the glass, because

the air which would have supported it has been withdrawn.

[Illustration: AIR IN THE EGG.]

[Illustration: DESCENDING SMOKE.]

THE SOUNDLESS BELL.

Set a bell on the pump-plate, having a contrivance so as to ring it at

pleasure, and cover it with a receiver; then make the clapper sound

against the bell, and it will be heard to sound very well; now exhaust

the receiver of air, and then when the clapper strikes against the sides

of the bell the sound can be scarcely heard.

[Illustration: SOUNDLESS BELL.]

[Illustration: FLOATING FISH.]

THE FLOATING FISH.

If a glass vessel containing water, in which a couple of fish are put,

be placed under the receiver, upon exhausting the air the fish will be

unable to keep at the bottom of the glass, owing to the expansion of

the air within their bodies, contained in the air bladder. They will

consequently rise and float, belly upwards, upon the surface of the

water.

THE DIVING BELL.

The diving bell is a pneumatic engine, by means of which persons can

descend to great depths in the sea, and recover from it valuable

portions of wrecks and other things. Its principle may be well

illustrated by the following experiment. Take a glass tumbler, and

plunge it into the water with the mouth downwards, and it will be found

that the water will not rise much more than half way in the tumbler.

This may be made very evident if a piece of cork be suffered to float

inside the glass on the surface of the water. The air within the tumbler

does not entirely exclude the water, because air is elastic, and

consequently compressible, and hence the air in the tumbler is what is

called condensed. The diving bell is formed upon the above principle;

but instead of being of glass, it is a wooden or metal vessel, of very

large dimensions, so as to hold three or four persons, who are supplied

with air from above by means of powerful pumps, whilst the excess of air

escapes at the bottom of the bell, as may be seen any day at the Royal

Polytechnic Institution.

[Illustration]

[Illustration]

EXPERIMENTS.

1. Place a cylinder of strong glass, open at both ends, on the plate of

the air-pump, and put your hand on the other end, and you will of course

be able to remove it at pleasure. Now exhaust the air from the interior

of the cylinder, and at each stroke of the pump you will feel your hand

pressed tighter and tighter on the cylinder, until you will not be able

to remove it: as soon as the air is again admitted to the interior of

the cylinder, the pressure within will be restored, and the hand again

be at liberty.

2. Tie a piece of moistened bladder very firmly over one end of a

similar glass cylinder, and place the open end on the plate of the pump.

As soon as you begin to exhaust the air from the interior, the bladder,

which was previously quite horizontal, will begin to bulge inwards, the

concavity increasing as the exhaustion proceeds, until the bladder, no

longer able to bear the weight of the superincumbent air, breaks with a

loud report.

3. The elasticity of air, or indeed of any gaseous body, may be shown by

introducing under the air-pump receiver a bladder containing a very

small quantity of air, its mouth being closely tied. As you exhaust the

air from the receiver, that portion contained in the bladder being no

longer pressed upon by the atmosphere, will gradually expand, distending

the bladder until it appears nearly full: on readmitting the air into

the receiver, the bladder will at once shrink to its former dimensions.

A shrivelled apple placed under the same conditions will appear plump

when the air is removed from the receiver, and resume its former

appearance on the readmission of the air.

4. There is a very pretty apparatus made for the purpose of showing the

pressure of the atmosphere, consisting of a hollow globe of brass, about

three inches in diameter, divided into two equal parts, which fit very

accurately together. It is furnished with two handles; one of them

screwed into a hollow stem, communicating with the interior of the

globe, and fitting on to the air-pump: the other is attached to a short

stem on the opposite side of the globe. In the natural state the globe

may easily be separated into its two hemispheres by one person pulling

the handles, but after the air has been exhausted from the interior it

requires two very strong men to separate the parts, and they will often

fail to do so. By turning the stopcock, and readmitting the air into the

interior of the globe, it will come asunder as easily as at first.

We are indebted to the weight of the atmosphere for the power we possess

of raising water by the common pump; for the piston of the pump

withdrawing the air from the interior of the pipe, which terminates in

water, the pressure of the atmosphere forces the water up the pipe to

supply the place of the air withdrawn. It was soon found, however, that

when the column of water in the pipe was more than thirty feet high, the

pump became useless, for the water refused to rise higher. Why? It was

found that a column of water about thirty feet high exerted a pressure

equal to the weight of the atmosphere, thus establishing an equilibrium

between the water in the pipe and the atmospheric pressure.

[Illustration]

This is the principle on which the barometer, or _measurer of weight_,

as its name imports, is constructed. The metal Mercury is about thirteen

and a half times heavier than water; consequently, if a column of water

thirty _feet_ high balances the pressure of the atmosphere, a column of

mercury thirty _inches_ high ought to do so also--and this is in fact

the case. If you take a glass tube nearly three feet long, and closed at

one end, and fill it with mercury; then, placing your finger on the open

end, invert the tube into a basin or saucer containing some of the same

metal; upon removing your finger (which must be done carefully, while

the mouth of the tube is completely covered by the mercury,) it will be

seen that the fluid will fall a few inches, leaving the upper part of

the tube empty. Such a tube with a graduated scale attached is in truth

a barometer, and as the weight of the atmosphere increases or decreases,

so the mercury rises or falls in the tube. This instrument is of the

greatest value to the seaman, for a sudden fall of the barometer will

often give notice of an impending storm when all is fine and calm, and

thus enable the mariner to make the preparations necessary to meet the

danger.

It was discovered by an Italian philosopher named Torricelli, and from

him the vacuum formed in the upper end of the tube above the surface of

the mercury has been called the Torricellian vacuum. It is by far the

most perfect vacuum that can be obtained, containing necessarily nothing

but a minute quantity of the vapour of mercury.

EXPERIMENT.

Pass a little ether through the mercury in the tube, and as soon as it

reaches the empty space it will boil violently, depressing the mercury,

until the pressure of its own vapour is sufficient to prevent its

ebullition. If you now cool the upper part of the tube, so as to

condense the vapour, the pressure being thus removed, the ether will

again begin to boil, and so alternately, as often as you please, in

order to show this fact with effect, the bore of the tube should not be

less than half an inch in diameter.

[Illustration]

EXPERIMENT.

To show that the heat abstracted by the boiling of one liquid will

freeze another, fill a tall narrow glass about half full of cold water

(the colder the better), and place in it a thin glass tube containing

some ether. Put them under the receiver of an air-pump. As you exhaust

the air, the ether will begin to boil, until at length, by continuing

the exhaustion, the water immediately surrounding the tube of ether

will freeze, and a tolerably large piece of ice may thus be obtained.

Ether evaporates so rapidly even under the pressure of the atmosphere,

that a small animal, such as a mouse, may be actually frozen to death by

constantly dropping ether upon it. If poured on the hand, it produces a

degree of cold that soon becomes, to say the least, unpleasant.

EXPERIMENT.

Place a flat saucer containing about a pound of oil of vitriol under the

receiver of the air-pump, and set in it a watch glass containing a

little water, supported on a stand with _glass_ legs. Exhaust the

receiver, when the water will evaporate, but without boiling; and the

vapour being absorbed as it forms by the oil of vitriol, the vacuum is

preserved, and the evaporation continues, until the vapour has

abstracted so much caloric from the remainder of the water that it is

all at once converted into ice.

In most elementary works on chemistry may be found a long table of

freezing mixtures, as they are called, some with and others without ice

or snow. We have selected a few from each division.

WITH ICE OR SNOW.

{ Snow or powdered ice 2 parts.

{ Powdered common salt 1 „

{ Snow 5 „

{ Powdered common salt 2 „

{ Powdered sal ammoniac 1 „

{ Snow 3 „

{ Dilute sulphuric acid 2 „

{ Snow 2 „

{ Crystallized muriate of lime 3 „

WITHOUT SNOW OR ICE.

{ Sulphate of soda 3 parts.

{ Dilute nitric acid 2 „

{ Nitrate of ammonia 1 „

{ Water 1 „

{ Phosphate of soda 2 „

{ Dilute nitric acid 1 „

{ Sulphate of soda 2 „

{ Muriatic acid 1 „

The effects of most of these mixtures may be considerably increased by

previously cooling the ingredients _separately_ in other freezing

mixtures.

In connexion with this branch of science, and especially with chemistry,

the youthful philosopher should practise the art of decanting air from

one jar to another standing over water, beginning by passing it from a

small to a larger jar, then with two of equal size; and when he can

accomplish the transfer without permitting even one bubble to escape, he

may essay the much more difficult task of transferring the air from a

large to a smaller jar.

[Illustration]

He should also practise using the blowpipe until he can keep up a steady

and uninterrupted flame for ten minutes or a quarter of an hour, without

stopping for breath. It is quite possible to replenish _wind_ in the

mouth, which alone ought to be used, without interrupting the breathing

for an instant, but it requires some practice.

[Illustration: (1) Box blowpipe, A, the stem; B, the mouthpiece; C, the

box, in which is placed a piece of sponge to absorb the moisture of the

breath; D, the extremity (this is moveable, and each blowpipe has

several, with different apertures); E, the hollow pivot, in which the

extremity and its arm turn. The box is opened by the milled ring seen at

the bottom.]

SPECTRUM ANALYSIS.

If the youthful student wishes to occupy his leisure in a most amusing

and instructive manner, and at the same time to acquire a ready method

of testing for the presence of the various metals, he cannot do better

than purchase from Mr. John Browning, of 111, Minories, the Amateur’s

Spectroscope, price two guineas; or, if sufficiently ingenious, a prism

and lenses may be purchased for sixteen shillings, and fitted up in

paper tubes. Indeed, it is always better to buy some complete set or

portion of a set of apparatus, instead of the trashy so-called “Boys’

Cabinets,” which contain very much that is useless and very little for

instructive experiments. The instrument enables the observer to

examine the properties of metallic salts when in the state of a glowing

or incandescent gas. Common light affords a continuous spectrum or band

of colours, red at one end and violet at the other, the intermediate

colours being orange, yellow, green, blue, and indigo. The spectra of

the metals in the state of luminous vapour are not continuous, but are

represented by distinct bands or lines of light.

[Illustration: Solar Spectrum, and Spectra of the Alkanes and Alkaline

Earths. Copied from the Original Drawings of G. KIRCHHOFF and R.

BUNSEN.]

Every element, metallic or otherwise, when converted into a luminous

gas, produces lines which are always found in the same part of the

spectrum, and are therefore unalterable in position. The instrument

(Fig. 1) is called the Spectroscope.

[Illustration: _Fig. 1._--THE AMATEUR’S SPECTROSCOPE.

A, Tube with knife edges; B, the prism; C, the telescope; D, the lamp,

supplied with gas by pipe E; F, stand carrying the platinum wire. The

wire may be held in the hand, but is more convenient when attached to a

stand.]

The instrument (Fig. 2) will exhibit the dark fixed lines in the solar

spectrum, and also the bright lines in the spectra of incandescent

metals.

[Illustration: _Fig. 2._--THE HERSCHEL-BROWNING DIRECT VISION

SPECTROSCOPE.]

[Illustration: _Fig. 3._

A, another form of gas lamp; B, the flame; C, adjusting stand to carry

the platinum wire and the lamp.]

HOW TO USE THE SPECTROSCOPE.

Screw the tube carrying the knife edges at the small end into the prism

bore, and the telescope into its proper ring. Now place any common

bright light exactly in front of the knife edges, and while looking

through the telescope arrange the knife edges until a bright and

continuous spectrum is visible.

TO OBTAIN THE BRIGHT LINES IN THE SPECTRUM GIVEN BY ANY SUBSTANCE.

Remove the bright flame from the front of the knife edges, and

substitute in its place the flame of a common spirit lamp or, still

better, a gas-jet known as a Bunsen’s Burner. Take a piece of platinum

wire about the substance of a fine sewing needle; bend the end into a

small loop about the eighth of an inch in diameter; fuse a small bead of

the substance, or salt, to be experimented on into the loop of the

platinum wire, and attaching it to any sort of light stand or support,

bring the bead into the front edge of the flame, a little below the

level of the knife edges. If the flame be opposite the knife edges, on

looking through the eye-piece of the telescope the fixed lines due to

the substance will be plainly visible. When minute quantities have to be

examined, the substance should be dissolved, and a drop of the solution,

instead of a solid bead, be used on the platinum wire.

The delicacy of this method of analysis is very great. Swan found, in

1857 (Ed. Phil. Trans., vol. xxi. p. 411), that the lines of sodium are

visible when a quantity of solution is employed which does not contain

more than 1/2,500,000 of a grain of sodium.

To view Fraunhofer’s lines on the solar spectrum it is only necessary to

turn the knife edges towards a white cloud, and make the slit formed by

the knife edges very narrow by turning the screw at the side of them. In

every instance the focus of the telescope must be adjusted in the

ordinary way, by sliding the draw tube until it suits the observer’s

sight, and distinct vision is obtained.

It should be noted that lines at various parts of the spectrum require a

different adjustment in focussing the telescope.

The small prism turning on a joint in front of the knife edges is for

the purpose of showing two spectra in the field of view at the same

time. To do this it must be brought close to the front of the knife

edges. Then one flame must be placed in the position in which the flame

of the candle is shown in the small figure, and the other directly in

front of the slit. On looking through the telescope, as before

described, the spectra due to the two substances will be seen one above

the other.

When the slit is turned towards a bright cloud, and a light is used in

the position of the candle flame, the spectrum of any substance may be

seen compared with the solar spectrum. In this manner Kirchhoff

determined in the solar spectrum the presence of the lines of the

greater number of the elements which are believed to exist in the sun.

PROFESSOR STOKES’ ABSORPTION BANDS.

This instrument is expressly adapted to the prismatic analysis of

organic bodies, according to the method recommended by Professor Stokes

in his lecture at the Chemical Society, printed in the _Chemical News_.

To observe these bands it is only necessary to place a very dilute

solution of the substance in a test-tube, then fix the test-tube in the

small clip attached to a ring which slips on in front of the knife

edges. Upon bringing any bright light in front of the tube, on looking

through the telescope, if the instrument has been properly adjusted, a

bright spectrum will be seen, interrupted by the dark bands due to the

substance in solution.

One of the simplest and most interesting experiments of this kind can be

made by preparing dilute solutions of madder, port wine, and blood.

In these very dilute solutions no difference can be detected by the

unassisted eye, but on submitting them in the manner already described

to the test of spectrum analysis, very different appearances will be

presented.

The absorption bands may, however, be most conveniently examined and

accurately investigated by means of Sorby and Browning’s new

Micro-Spectroscope.

TO MAP OUT ANY SPECTRUM.

Place the eye-piece with cross wires in the telescope, with the cross in

the shape of an [X]. Then move the telescope so that the point where the

wires cross comes successively in contact with the various lines, noting

the readings of the nonius on the arc. From these readings, by the help

of any mechanical scale of equal parts, a map may be easily constructed.

By the assistance of spectrum analysis four new metals have been

discovered, viz. Cæsium, Rubidium, Thallium, Norium. It is often a cause

of grumbling with young beginners in chemistry that there is so much

washing of glasses and slopping required to obtain proper results from

tests; but here is a most refined mode of testing, which may be carried

on in the drawing-room, and so delicate that a portion of sodium salt

even less than 1/180,000,000th part of a grain can be easily detected.

Formerly the metal Lithium was considered to be a very rare one, but

since the use of the spectroscope, quantities less than the

1/6,000,000th of a grain have been detected; it now appears that lithium

is a very common metal, and exists in the human body, the sea, and may

be obtained from many other common sources. Lithium has been found in

the ashes of marine plants, the ashes of tobacco--in milk, coffee, tea,

human blood, and in muscular tissue; it has also been found in meteoric

stones.

The coloured plate is a faithful copy of Kirchhoff’s Diagram, and shows

the solar spectrum, also the spectra of potassium, sodium, lithium,

strontium, calcium, barium, rubidium.