[Ill.u.s.tration: 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 pa.s.sing 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 ma.s.s. 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.

[Ill.u.s.tration: 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 s.p.a.ce 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 pota.s.sae," 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.

[Ill.u.s.tration: 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 ill.u.s.tration 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.

[Ill.u.s.tration: 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 b.u.t.terfly'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.

[Ill.u.s.tration: ANTENNae OF FLY.]

Take the ANTENNae 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 ill.u.s.tration is given part of an antenna of the common blue-bottle fly, in order to show the remarkable cavities which exist within the antennae, and which are thought by some anatomists to be organs of hearing, and by others to be organs of smell.

[Ill.u.s.tration: 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 ill.u.s.tration 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 ill.u.s.tration.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: BIRD'S-HEAD PROCESS.]

[Ill.u.s.tration: NOCTILUCA.]

[Ill.u.s.tration: PEDICILLARIae.]

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 phosph.o.r.escence 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 pedicillariae, and a sketch of them is given in the ill.u.s.tration.

[Ill.u.s.tration]

OPTICS AND OPTICAL AMUs.e.m.e.nTS.

"'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.

[Ill.u.s.tration]

REFRACTION.

Bodies which suffer the rays of light to pa.s.s through them, such as air, water, or gla.s.s, 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 gla.s.s L G is bent from the point C, instead of pa.s.sing in the direction of the dotted line. But if the ray F C falls perpendicularly on the gla.s.s, 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.

[Ill.u.s.tration]

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.

[Ill.u.s.tration]

THE MULTIPLYING GLa.s.s.

The multiplying gla.s.s is a semicircular piece of gla.s.s cut into facets or distinct surfaces; and in looking through it we have an ill.u.s.tration 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 gla.s.s.

[Ill.u.s.tration]

TRANSPARENT BODIES.

Transparent bodies, such as gla.s.s, may be made of such form as to cause all the rays which pa.s.s 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.

[Ill.u.s.tration]