Image unavailable: MUSSEL-SHELL. EARWIG. LOBSTER-CLAW. SUGAR-TONGS. PINCERS.MUSSEL-SHELL. EARWIG. LOBSTER-CLAW. SUGAR-TONGS. PINCERS.
Then we have the still smaller and feebler Pincers of civilised life, such as the Sugar-tongs and the ordinary Coal-tongs of our firesides. Anatomists could have had no practical existence without the Pincers, of which their beautifully constructed and much-elaborated forceps are but variations.
Take, again, the dentist, with his series of shining instruments, which he so carefully keeps out of sight until he has got his patient safely in that awful chair, and which glide, as by a conjurer’s trick, empty into an open mouth, and return in a few seconds with a tooth between their polished jaws.
Allthese instruments have their parallels in Nature, and in many instances the natural pincers might supply useful hints to modern tool-makers.
In the left-hand upper corner of the illustration is shown the common fresh-water Mussel, which is so plentiful in almost all our rivers and many of our ponds. Its scientific name isUnio margaritiferus. The latter title, which signifies “pearl-bearing,”is given to it because it furnishes the British pearls which were at one time so highly valued.
Like other bivalve molluscs, this Unio has the two halves of the shell fitting quite tightly upon each other, and, when they are drawn together by the contraction of the internal muscles, they can give a very severe pinch. In many uncivilised parts of the world the natives take advantage of this property, and use them as tweezers, chiefly for the purpose of pulling out hairs which they are pleased to think are not needed.
I need not state that with all bivalves the power is increased in proportion to the size of the shell. Even an Oyster can pinch most severely, while the Giant Clam, the shell of which weighs some four hundred pounds, could nearly take off a man’s leg if it seized him.
Mr. J. Keast Lord, in his “Naturalist in British Columbia,” relates an amusing story that was told to him by an old settler respecting the power of the Clam’s grip:—
“You see, sir, as I was a-cruising down these flats about sun-up, the tide jist at the nip, as it is now, I see a whole pile of shoveller-ducks snabbling in the mud, and busy as dogfish in herring time. So I creeps down, and slap I let ’em have it. Six on ’em turned over, and off went the pack, gallows scared, and quacking like mad.”
“Down I runs to pick up the dead uns, when I see an old mallard a-playing up all kinds o’ antics, jumping, backing, flapping, but fast by the head, as if he had his nose in a steel trap; and when I comes up to him, blest if a large Clam hadn’t hold of him, hard and fast, by the beak.”
“The old mallard might ha’ tried his hardest, but may I never bait a martin-trap again if that Clam wouldn’t ha’ held him agin any odds till a tide run in, and then he’d ha’ been a gone shoveller sure as shooting. So I cracked up the Clam with the butt of my old gun, and bagged the mallard.”
Of course the reader will remember that this was only an ordinary Clam, and not one of the giant race.
Belowthe shell are two very perfect instances of natural Pincers, each acting in a different manner, but on the same principle.
The Earwig is too familiar to need much description, but I may as well state that its pincers are not primarily intended as weapons, although they can be so used on occasion. (I was about to say, at a pinch, but refrain.) They resemble our ordinary pincers in that both blades move equally, and they are so completely under the control of their owner, that the insect uses them with a delicacy of touch that a lady’s fingers could hardly surpass. They are really tools, and not weapons, and are employed for the purpose of folding the wide and delicate wings under the tiny elytra.
There is another insect called the Scorpion-fly (Panorpa), the male of which is furnished with a pair of pincers at the end of a long and flexible tail, articulated just like the tail of a scorpion, and moved in exactly the same manner. It is but a little insect, but its gestures are so menacing as it flourishes its tail about, that non-entomologists may well be pardoned for being afraid of it. Moreover, small as are the pincers, they really can give a smart nip, and make themselves felt on the human skin.
Ifwe want examples of exceedingly powerful pincers, we need only go to the Lobsters and Crabs, especially to the latter, whose claws are often of enormous thickness in proportion to the size of the animal. All those who have visited the seaside know how severe is the pinch of the common Green Crab, comparatively small though it be, and the same may be said of the river crayfish, which is, in fact, a lobster in miniature.
As to the lobster itself, fishermen are so well acquainted with the power of its claws, that they tie them together with string as soon as the animal is caught. Formerly they used to “peg” them,i.e.drive a wooden peg into the joint so as to prevent it from moving. This custom, however, is now prohibited by law on account of its cruelty.
The power of the Crab’s claws is so great that a bite from a large Crab will inflict a severe injury, and render a hand helpless. It has more than once happened that men who have been feeling for Crabs in the recesses of the rocks at low water have been seized, and seriously imperilled, not being able to release themselves from the gripe.
Indeed, it is said that there have been instances where theCrab has held so tightly, that the man has been drowned by the returning tide, no one having come to his assistance. I am, however, inclined to doubt this statement, thinking that the Crab would not be likely to remain in its hiding-place very long after the water came up. Still, that such an idea should be currently believed in many parts of England shows the estimation in which the gripe of the Crab’s claw is held.
Files and Sand-papers.—The Sheffield File and its Structure.—The Equisetum, Mare’s Tail, or Dutch Rush.—Beauty of its Surface when seen through the Microscope.—Sand-paper.—Skin of Dog-fish, Skate, and Shark.—Skate-skin used for Sword-handles.—Distinction between the File and Sand-paper.—Measuring Tools.—The Plumb-rule and the Level.—Their Use in Tunnelling.—The Measure and its Uses.—The Two-foot Rule and the Tape Measure.—Ovipositor of Gall-fly.—Tongues of the Woodpecker, Wryneck, and Creeper.—The Spirit-level and its Uses.—Theodolite and Callipers in Nature and Art.—The Contouring-glass.—Pincers of Earwig again.—Jaws of Insects.—The great Sialis of Columbia.
Files and Sand-papers.—The Sheffield File and its Structure.—The Equisetum, Mare’s Tail, or Dutch Rush.—Beauty of its Surface when seen through the Microscope.—Sand-paper.—Skin of Dog-fish, Skate, and Shark.—Skate-skin used for Sword-handles.—Distinction between the File and Sand-paper.—Measuring Tools.—The Plumb-rule and the Level.—Their Use in Tunnelling.—The Measure and its Uses.—The Two-foot Rule and the Tape Measure.—Ovipositor of Gall-fly.—Tongues of the Woodpecker, Wryneck, and Creeper.—The Spirit-level and its Uses.—Theodolite and Callipers in Nature and Art.—The Contouring-glass.—Pincers of Earwig again.—Jaws of Insects.—The great Sialis of Columbia.
HAVING now examined the analogies between the cutting, boring, striking and grasping tools of Nature and Art, we come to those finishing tools which smooth and polish the surface.
The first is the File, an instrument which needs but little description. It consists of a surface of hardened steel, broken up into rough-edged teeth of infinite variety, according to the work which the file has to do. It is rather remarkable, by the way, that at present the English files are infinitely superior to those produced in any other part of the world; that their teeth are all made by hand; and that a genuine Sheffield file will first cut its way through a piece of iron in half the time that would be occupied by a file of any other nation, and then would easily cut its antagonist in two.
Aslong as the File is intended to work upon metal, there is little difficulty in its manufacture, except that no machinery has yet been invented which can give the peculiar edging ofthe ridges, and to which is owing the unmistakable “bite” of a real English file.
But there are occasions when the hand of the most cunning file-maker is baffled, and when it is necessary to cut files so delicate that the unaided human eye cannot trace their teeth. Art, therefore, has recourse to Nature, and the cabinet-maker, who cannot obtain any file made by human hands which will answer his purpose in the higher branches of his trade, makes great use of the “Dutch Rush,” as he calls it. It is not a rush at all, but simply a species of Mare’s Tail, or Equisetum, a plant which fills in profusion almost every marshy spot in England.
Image unavailable: EQUISETUM. FILE.EQUISETUM. FILE.
The peculiar fitness of the Equisetum for this purpose cannot be appreciated even by those who use it until it has been viewed under the microscope. I have now before me a small piece of Equisetum, placed under a half-inch power, and viewed by direct illumination, it being treated as an opaque object.
The microscope reveals at a glance the source of the power which the ingenuity of man has taken advantage of. The surface of the Equisetum is seen to be composed of myriads of tiny parallel ridges, each ridge bristling with rows of flinty spicules, looking very much like the broken glass upon the top of a wall. Minute as they are, these spicules can do their work, and they enable the joiner to finish off work in a manner that could not be accomplished by any tool made by human hands.
I find, by recent inquiries, that modern joiners scarcely, if ever, use the Equisetum, preferring emery-paper as cheaper and more expeditious, and knowing that the popular eye is not able to appreciate the difference of the surface obtained bythe Equisetum from that which is given by the finest emery-paper ever made. Wood-carvers, however, if they be of the conscientious kind, and love their work for its own sake, adhere to the Dutch Rush, and are all the happier for it.
Passwe now to the coarser kinds of polishers, the chief of which is popularly known as Sand-paper, and is made by coating some tissue with glue, and scattering upon it sand of different qualities, according to the work to be done. Sometimes, when the work is rough, the sand is large, rough, and coarse, and sometimes, when the work is fine, the sand is so carefully sifted before it is scattered on the glued paper, that there is little distinction between the sand-paper and emery-paper. Linen, by the way, is generally used instead of paper, as being more enduring, less liable to crack, and capable of being folded so as to obtain access to crevices which paper could not touch.
Image unavailable: DOG-FISH SKIN, MAGNIFIED. SAND-PAPER, MAGNIFIED.DOG-FISH SKIN, MAGNIFIED. SAND-PAPER, MAGNIFIED.
Againin Nature we find a parallel, and the coarse Sand-paper of modern Art has long been anticipated in the scale-clad skins of many fishes.
The accompanying illustration is taken from the skin of a Picked Dog-fish found by myself lying dead on the rocks in Bideford Bay. I cut off a piece for transmission to the draftsman, and found that not only did it feel exactly like cutting through a piece of very common sand-paper, but that it blunted the edge of a new knife in exactly the same manner as would have been done by the roughest of sand-paper.
This kind of skin is common to all the shark tribe (including the Dog-fishes, which are but sharks in miniature), and to theSkate, Saw-fish, &c. I have now before me a small, but perfect example of the Saw-fish, the surface of which is covered with flinty scales like those of the Dog-fish, but very much smaller, requiring the aid of a magnifying lens to distinguish them. Even to guess at the number of them is impossible, for they cover the whole of the body, and extend to the very end of the beak, in some places glittering in a strong light as if pounded glass had been sprinkled all over the fish. One of the most interesting points in their structure is the manner in which they reach the rounded jaws, and there become converted into teeth powerful enough to crush the animals on which the fish live. The structure of these jaws will be explained in a future chapter.
Some of the skates and sharks have these scales of great size, so as to show their formation almost without the aid of a magnifying-glass. This is the case with a species of skate, the skin of which is used by the Japanese for wrapping round the handles of their best swords, and which is greatly valued by that nation, the sword being an almost sacred article in the eyes of a Japanese.
There is a well-known museum in which these swords are labelled as having handles of “granulated ivory.” Now, in the first place, there is no such thing as granulated ivory; and, in the next, a mere glance ought to tell the observer that the so-called ivory is a skin of some sort, worked upon the handle while wet, and kept in its place by copper studs. Even the junction of the edges is perceptible, and yet the authorities of the museum in question, although they have been repeatedly corrected, still persist in calling the skate-skin by the absurd title of granulated ivory.
However, if ivory could be granulated, it would certainly look very much like the skate-skin. When examined closely, the scales, whether of Dog-fish, Skate, Shark, or Saw-fish, are seen to resemble hexagonal cones, not coming quite to a point, but truncated, so as to have an hexagonal flattened tip. They are almost of a flinty hardness, especially at their tips, and on inspection of them the observer is not surprised at the use of Dog-fish skin in place of sand-paper.
Perhaps the reader may ask why the Equisetum should be taken as the prototype of the file, and the skin of the Dog-fishas that of sand-paper. The reason is this. The flinty points of the Equisetum are set upon parallel ridges something like those of a file, while the scales of the Dog-fish are without any apparent order, being crowded against each other like the cutting particles upon the sand-paper. That there should not be an order, and that a definite one, is out of the question. But it has not yet been detected by human eyes, and therefore may be practically treated as non-existent.
Inmany of the arts, more especially those which belong to engineering and carpentering as a part of architecture, it is absolutely necessary to make sure of a perpendicular line,i.e.a line which, if continued, would reach from any point of the earth’s surface to its exact centre below and its zenith above. Were it not for the power of producing this line, none of the great engineering works of modern or ancient days could have been undertaken.
Take, for example, the wonderful tunnels which have been driven through the earth, of which the Mont Cenis Tunnel is one of the greatest triumphs of modern engineering. Beginning, as the workmen did, at opposite ends of a tunnel many miles in length, and labouring only by the lines laid down by the engineers, the men worked steadily on until they met in the centre.
A few blows, and the then narrow dividing wall was shattered, the men shook hands through the aperture, and then, after enlarging it, leaped wildly from one side to the other, having successfully solved the great problem. With such marvellous precision had the lines been laid, that only a few inches had to be smoothed down on either side, and the sides or walls of the tunnel showed no traces of the junction.
So rapid has been the progress of engineering that a tunnel of a mile in length would, within the memory of man, have been thought as daring a project as was the Mont Cenis Tunnel, which has just been given as an example. Indeed, I know of a railway tunnel, not quite a mile in length, where the engineers had committed some error, so that the two halves, instead of meeting exactly, overlapped each other so muchthat the mistake was only discovered by the workmen, who heard the strokes of their companions’ picks on their sides, and not in front. Consequently, a great waste of time took place, and the centre of the tunnel had to be made with a double curve, like the letter S, and trains are obliged to slacken speed until they have passed it.
Those who have lived long enough to remember the current literature of the past generation will call to mind the ridicule that was cast upon the idea of a tunnel that should pass under the Thames. That it would be useful if it could be completed, no one ventured to doubt, but that such an idea could be conceived by any one out of a lunatic asylum was rather too much for the journalists of the day. However, the tunnel was made, and so proved the theorists wrong on the one side. And, when made, it was of very little use, which proved them wrong on the other side. Now the proposal to carry a submarine tunnel from England to France excites not half the opposition that was elicited by the comparative child’s-play of a tunnel under the Thames.
The only mode of laying down the lines on which the men worked is by suspending very heavy balls to very fine wires, and then, by means of delicate optical instruments, ascertaining whether the wires are in line with each other.
Familiar instances of the use of this principle may be seen in the plumb-rule and level of the builder or carpenter. The latter, with a base of ten feet in length, is often used by the gardener when he wishes to lay the absolutely level lawns that are required for our modern game of croquet, where the hoops are scarcely wider than the balls, and the lawn has in consequence to be nearly as level as a billiard table.
I may here remark that the name plumb-rule is derived from the Latin wordplumbum, or lead, in allusion to the leaden weight at the end of the string. The word “plumber” is due to the same source, and signifies a worker in lead.
Theseinvaluable aids to the development of civilisation are due to one principle, namely, that which we call Gravitation, but which ought more properly to be termed Attraction, and which attracts all parts of the earth towards its centre. We are all familiar with the anecdote of Newton and the fallingapple, which may be true or not, but which at all events bears on the present subject. No matter on what portion of the spherical earth a tree may be, every fruit becoming disengaged from it is attracted to the earth, the line which it takes, unless disturbed by external forces (such as wind, &c.), being that which passes from the zenith to the centre of the earth.
Image unavailable: FALLING FRUIT. PLUMB-RULE. LEVEL.FALLING FRUIT. PLUMB-RULE. LEVEL.
This imaginary line is a perfect perpendicular, and the visible line which is formed by the delicate wire of the tunnel-boring engineering instrument, or the comparatively coarse string of the plumb-rule and level, are approximations sufficiently close for practical purposes. So it is in a mathematical proposition. As mathematical lines have no breadth, they are simply indicated or represented by the lines of the figure, the bodily eye being incapable of seeing what is perfectly visible to the mental eye, namely, length without width. So the wire and string perform in practical work exactly the same office which is fulfilled by the lines of a mathematical proposition drawn on paper.
We have already, when treating of the Fall-trap, seen how this principle is brought into operation by those who are utterly incapable of discerning the physical principle, though they can apply it materially with wonderful effect.
Itis, perhaps, needless to mention the value of the Measure to any handicraftsman.
I well remember that when, some twenty-four years ago, I was taking lessons from a carpenter in the art of making ladders, gates, fences, hurdles, and other rough-and-ready work, my quaint old tutor related an anecdote of and against himself.He very ingeniously set me to work at boring the auger-holes in the gate-posts which were to be united by the mortise chisel and mallet, and to sweeten the rather severe, because unaccustomed, labour, told me that, when he was a boy, he was doing just the same thing.
Being rather tired of twisting the auger handle (and no wonder either), he withdrew the instrument, and put his finger into the hole by way of ascertaining its depth. Immediately he found himself on his back, having received a tremendous box on the ear from his father, whose parental wrath was excited by the idea of his son condescending to use his finger by way of measure, when he had a two-foot rule in its own special pocket.
There are, however, many cases where even a two-foot rule would be insufficient for the work, and where a measure of thirty or forty feet is needed.
Now, there is no doubt that by means of a two-foot, or even a six-inch, rule any number of feet might be measured accurately; but, considering the number of junctions that have to be made, it is not likely that any pretence to accuracy could be insured.
Then, a rod of forty, or even of twenty, feet in length would be awkward and unmanageable, and the only plan left is to take a string or cord of the requisite length.
Even here, however, is a difficulty. The string would not allow of short measurements, such as inches, being written upon it. Let, however, a broad tape of inelastic material be substituted for the string, and all is easy enough.
The next plan is to provide for the portability of the tape in question, to insure its reduction into the smallest possible compass, and to be sure that it is not twisted so as to damage its accuracy. These objects are all attained by the ordinary Tape Measure of the present day, which, whether it be a yard measure in a lady’s workbox, or a surveyor’s measuring tape, is a ribbon of comparatively inelastic material, coiled up when not wanted, and capable of being drawn out to its fullest extreme when needed.
Putting aside the breadth of the line, and consequently disregarding the liability to twist, we have in the Fishing-reel of the modern angler an exact case in point. So we have inthe lady’s yard measure, and in the gardener’s or builder’s tape, all these being modifications of the same idea.
Image unavailable: OVIPOSITOR OF GALL-FLY. SPRING MEASURE.OVIPOSITOR OF GALL-FLY. SPRING MEASURE.
Suppose now that we pass to Nature, so as to ascertain whether any such provisions were in existence before it was imitated, however unconsciously, by man. This certainly was the case with one of the commonest and most insignificant of our insects, the little Gall-fly, belonging to the genus Cynips. It could not lay its eggs without the aid of a very long ovipositor, and, owing to structural details, it cannot carry that ovipositor in a straight line, as is done by many insects, some of which have already been mentioned. Accordingly, it is coiled up exactly like our measuring tapes, and can be unrolled when needed. The long, protrusible tongues of the Wryneck, Creeper, and Woodpecker are examples of a similar structure, the tendinous portions being coiled round the head when not needed.
Havingnow seen how the forces of Nature enable us to produce a perfectly perpendicular line, we will see how the same force, though applied in a different manner, enables us to produce a perfectly horizontal line, the intersection of the two lines producing a right angle.
Image unavailable: FLOATING BUBBLE. SPIRIT-LEVEL.FLOATING BUBBLE. SPIRIT-LEVEL.
The measuring tool in question is called the Spirit-level, and is represented on the right hand of the accompanying illustration. Its construction is very simple, consisting of a tube, nearly filled with spirit, and having just one bubble of air in it. Now, owing to the force of gravitation, the air-bubble must always be uppermost. Consequently, if the tube be a perfect cylinder, whenever it is held so that the bubble isin the centre, the tube must be horizontal, a hair’s breadth of deviation altering the line. I may here mention that, as far as the principle of the instrument goes, water would serve the purpose as effectively as spirit. But as in cold weather the water might freeze, and so burst the tube, as well as being useless until it was thawed, spirit is always substituted.
This instrument is used for various purposes. Sometimes it is employed for levelling billiard tables, or for ascertaining the exact level of walls and other parts of buildings. Surveyors could scarcely do their work without the Spirit-level, which forms an important part of their chief instrument, the theodolite. Indeed, the new science of land drainage, by which the tough, unproductive clay soil is converted into fertile earth, is entirely dependent on the use of the Spirit-level, which detects the slightest rise or fall in the ground.
A most ingenious modification of the Spirit-level is used by military engineers, and is known by the name of the “Contouring-glass,” a term which requires some explanation.
It is of the utmost importance that a military engineer should be able, whether on foot or on horseback, to ascertain the approximate heights of the various points which he visits, the efficiency or failure of a battery very much depending on the comparative elevation of the spot on which the battery is placed, and that of the place against which its fire is directed. In an unknown country, of which no detailed maps exist, an invading force must of necessity depend on the extemporised surveys of their engineer officers, and one of the most valuable of their devices is the system of Contouring, invented, as far as I know, by the late Colonel Hutchinson, R.E.
The idea is simple enough. A hill is seen, and the engineer makes a sketch of it before he ascends. At the foot he halts, and marks the spot where his foot presses the earth. He then looks in front at a spot exactly on the level of his eye, marks it, and walks to it. He then draws a line across his sketch, at the exact spot on which he is standing, and that is the first “contouring line.” Others follow, until he has reached the top of the hill.
Now, if he can trust himself to look exactly horizontally, he has ascertained the elevation of every part of the hill. He knows the height of his eye from the sole of his foot, andcalculates accordingly. Suppose, for example, that it be five feet, and that ten contouring lines are marked, he knows that the entire height is fifty feet, and that each line means an elevation of five feet.
This is a very excellent theory, but one which is not reduced to practice so easily as it looks. There is nothing more deceptive than a contour, especially upon an irregular hill, the invariable mistakes being either greatly to overrate or underrate the height of the contour. When I took my first lesson in this art I caused much amusement to the professor under whom I was studying, by making Shooter’s Hill consist of about seventeen contours. However, as many military students made very much the same mistake, I was not so humiliated as I supposed.
Of course, if a surveying officer be mounted, he takes the contour line as measured from his eye to the ground through the centre of the saddle.
After some practice the eye becomes so much accustomed to the contouring lines that they are taken almost mechanically; but, until this result be gained, an absolute proof is needed, which is furnished by the Contouring-glass—which, by the way, is not a glass at all, after the common acceptation of the word.
It is a simple brass tube about three inches long, not thicker than a man’s little finger, and open throughout. A small spirit-level is fixed on its lower surface, and on the very centre of the upper surface is a tiny steel mirror, which projects downwards like a knife-blade. In order to get a “contour,” the observer looks through the tube, slightly depressing its end. He then gradually raises it, still looking through it. As the tube becomes exactly horizontal the bubble in the spirit-level is reflected in the little mirror, and the object on which the tube is directed is in consequence on a level with the observer’s eye.
At first the management of the contouring-glass is rather tedious; but after a little practice it can be used without pausing for a single step.
Invaluableas is the Spirit-level, with its various modifications, it is nothing but an adaptation of that natural law which causes the bubbles to float on the surface of a stream instead ofbeing submerged below it. We have all seen the multitudinous bubbles of soda-water, or of any effervescing liquid, and have noticed how they are very small when generated, but enlarge quickly, and rise to the surface with a rapidity equal to their enlargement. The same phenomena may be observed in any water-fall, or even in the very familiar and unpoetical operation of pouring beer from a jug into a glass.
The reader will see that in the plumb-rule, the level, and the spirit-level one single principle is employed, namely, the attraction of matter towards the centre of the earth. In the two former instruments this attraction gives a vertical line, and in the latter it gives a horizontal line, but the principle is the same in both.
Image unavailable: JAWS OF SIALIS. CALLIPERS.JAWS OF SIALIS. CALLIPERS.
We conclude the history of measuring tools with the Callipers. For ordinary purposes, and upon a plane surface, the Compasses answer every purpose. But there are various arts, especially sculpture, in which the compasses, with their straight legs, are absolutely valueless, and their place must be supplied by a differently shaped instrument. For example, no ordinary compasses could measure the exact distance from the nostril to the back of the head, or even touch two points at opposite sides of a limb, and it is therefore necessary to have compasses withcurved legs. These are termed Callipers, and can be used on a plane as well as on a rounded surface.
NaturalCallipers are plentiful enough, and may be found extensively among the insect tribes. There are, for example, the pincers of the Earwig, which have already been described onpage 259, and which are, in the common species, formed exactly like the Callipers of the sculptor.
Then we have various insect jaws, especially those of the carnivorous species, one of the most curious being the large insect which is shown in the illustration, upon a very reduced scale. In the male the jaws are exceedingly long and curved, as may be seen by reference to the illustration. I have now before me a pair of sculptor’s callipers, and the resemblance between them and the jaws of the Sialis is strangely close, the curve being almost exactly the same in both cases.
The scientific name of this insect isSialis armata, and it is a native of Columbia.
The Camera Obscura.—Telescopes, Microscopes, and Spectroscopes, and their separate Objects.—Structure of the Camera Obscura.—The Double Convex Lens.—Its Use as a Burning-glass.—The Meridian Gun in Paris.—Signification of the Word “Focus.”—The Human Eye and its Analogies to the Camera Obscura.—Forms of various Lenses.—Long and Short Sight.—Their Causes and Means of Remedy.—Alteration of Sight in the Diver.—Long and Short sighted Spectacles.—The Eye of Birds.—Its beautiful Structure.—Washing-glasses and the “Nictitating” Membrane.—Combination of Images.—Natural Stereoscopes.—The Pseudoscope and its Effects on an Object.—The Multiplying-glass.—The Eight Eyes of the Spider and their Arrangement.—The Seventy Thousand Eyes of the Butterfly.—Form of the Facets.
The Camera Obscura.—Telescopes, Microscopes, and Spectroscopes, and their separate Objects.—Structure of the Camera Obscura.—The Double Convex Lens.—Its Use as a Burning-glass.—The Meridian Gun in Paris.—Signification of the Word “Focus.”—The Human Eye and its Analogies to the Camera Obscura.—Forms of various Lenses.—Long and Short Sight.—Their Causes and Means of Remedy.—Alteration of Sight in the Diver.—Long and Short sighted Spectacles.—The Eye of Birds.—Its beautiful Structure.—Washing-glasses and the “Nictitating” Membrane.—Combination of Images.—Natural Stereoscopes.—The Pseudoscope and its Effects on an Object.—The Multiplying-glass.—The Eight Eyes of the Spider and their Arrangement.—The Seventy Thousand Eyes of the Butterfly.—Form of the Facets.
HISTORY seems to fall into natural divisions, and to write the records of time in successive epochs, recording the advance of the human race. Some of them have apparently disappeared except by the strange relics which they have left behind, but though nothing is known of the men who worked in these ancient times, they stamped their mark upon the earth, and evidently left the world better than they found it.
A very admirable treatise on this subject has been written by the late Rev. J. Smith, called the “Divine Drama of Creation.” In this work he divides the progress of the human race into five acts, like those of a drama. The first act is the Hebrew Mission, the second the Greek Mission, the third the Roman Mission and the Middle Ages, the fourth the National Mission, and the fifth the Universal Mission.
Certainly a scene of the last act is now in progress, and may be entitled the Scientific Mission. The last hundred years have been indeed the age of discovery, and, during that time,the life of civilised man has been quite altered, so that practically his sojourn upon earth has been doubled. Steam, with all its various applications, electricity, and other kindred arts have become so intermingled with our lives, that it is difficult to imagine what our state would be if we were suddenly and utterly deprived of them. The loss to all would be incalculable, and not the least of the losses would be that of ready communion with our fellow-creatures.
Of these arts we will now take that which is named at the head of this division of the book, and see how far it is a development of natural facts.
I havealready spoken of arts as being akin to each other. They are more than this, and every day of the world’s progress teaches us that Art, Science, and Manufacture are sisters, all born of one family, and all depending mutually on each other.
Take, for example, our present theme—namely, Optics—and see how dependent it is upon Manufacture and Art. Without the former, man could not construct those beautiful telescopes, microscopes, spectroscopes, of the present day, which are evidently but the precursors of instruments which will work still greater marvels.
The first enables us to see solar systems without number, to which our own, vast as it seems to us, is but as a grain of sand in the desert. The next instrument makes revelations as marvellous of the infinitely minute as does the telescope of the infinitely great, enabling us to see living organizations so small that thirty-two millions could swim in a cubic inch of water. The third, a comparatively modern instrument, reveals the composition of objects, and can detect and register the materials of which the sun and fixed stars are made, or detect an adulteration in wine. It can adapt itself equally to the telescope and microscope, and the very same instrument which will reveal the character of an invisible gas in the Pole-star, when attached to the telescope, can, when connected with the microscope, point out the presence of half a corpuscle of blood where no other instrument could discover any trace of it.
All these instruments, together with many others, will be described in the present division of the work, and their analogies with Nature shown.
Wewill now take the subject of the Camera Obscura, an instrument with which the photographic apparatus of the present day has made most of us familiar. As its action depends chiefly upon the glass, or lens, through which the rays of light pass into the instrument, we will first explain that.
A “lens” is a glass formed in such a manner that the rays of light which pass through it either converge to a focus, or are dispersed, by means of the law of refraction. Every one who has been photographed—and who has not?—will remember that when the sitter has taken his position, the photographer brings to bear upon him a circular glass fixed into a short tube, and then looks through the instrument as if he were taking aim with some species of firearm. It is no matter of wonder that when savages see the photographic camera for the first time they are horribly frightened, for there is really something weird-like in the appearance of the lens thus presented.
Now, this lens is of the shape called “double convex,” both sides being equally rounded, so that a section of it would be shaped very much like a parenthesis (). The effect of this form of lens is to bring the rays of light to a point at a given distance from the centre. This point is called the “focus,” and is well known by means of the common burning-glass, which will set fire to objects placed in its focus, while itself remains quite cool.
I have seen lead pour down like water when placed in the focus of a large burning-glass, and even the harder metals will yield to the power of the sun’s rays when thus concentrated.
There is nothing which gives a more vivid idea of the amount of heat thrown on the earth by the rays of the sun than the effects of a moderately large burning-glass—say one of six inches in diameter. If we trace a circle of this size on the surface of the earth, it does not seem as if any very great amount of heat can be received, but when we catch the rays of that circle in our glass, and bring them together upon thefocus, the amount of heat can be appreciated. The well-known meridian gun in the Palais Royal is fired by the sun. A burning-glass of no very great size is placed over the touch-hole of the gun, with which its focus coincides. The lens is turned in such a manner that, as the sun attains the meridian, its rays are thrown upon the touch-hole, and consequently fire the gun.
The wordfocusis the Latin term for a domestic hearth, and is used in allusion to the heat which is manifested at the point on which the rays of the sun converge.
It is evident that, after reaching the focus, the rays, if they be not intercepted by some object, will cross each other, and form a large image, but reversed. This part of the subject will presently be explained.
Theaccompanying illustration shows two figures, one representing the section of a double convex lens made by the hands of man, and the other that of a double convex lens as seen in Nature.
Image unavailable: CRYSTALLINE LENS OF HUMAN EYE. DOUBLE CONVEX LENS.CRYSTALLINE LENS OF HUMAN EYE. DOUBLE CONVEX LENS.
The former has already been explained. The latter is the double convex lens of the human eye, by means of which the images of external objects are conveyed to the brain. Whenever this lens becomes thickened by disease, the sight is gradually dimmed, and at last total blindness is the result. This disease is popularly called “cataract,” and until late days was incurable. Now, however, any good oculist will attack a cataract, and either partially or entirely restore the sight. This operation is performed by carefully removing the convex lens, and supplying its place with a glass lens, which throws the rays of light on the same focus.
The figure shows the double convex lens of the human eye in its place.
Havingnow seen something of the properties of the double convex lens, we will examine its application to the Camera Obscura.
The lens is placed on one side of the camera, and is so made that it can be slid backwards and forwards, and the focus altered at will. The camera itself is a box completely closed, so that no light can enter it except that which passes through the lens. The latter is so arranged that the rays which pass through it are crossed, and throw their image on the opposite side of the camera. In the photographic camera a piece of ground glass is placed at the end, so that the rays fall upon it, and the operator can see whether the image is a good one. Of course the figures are reversed, so that the sitter seems to be on his head, but that is a matter of no consequence. Exactly the same effect is produced by the marine telescope.
Image unavailable: EYE AND IMAGE. CAMERA OBSCURA AND IMAGE.EYE AND IMAGE. CAMERA OBSCURA AND IMAGE.
The general structure of the camera is shown in the illustration, all needless details being omitted.
I may here remark that the term “camera obscura,” or dark chamber, alludes to the fact that the box is completely closed, and, but for the rays which pass through the lens, would be absolutely dark.
Theopposite illustration shows the most perfect camera obscura that can be imagined, namely, the human eye. Here we have a dark chamber, a double convex lens, and an image falling upon the back. Here the optic nerve comes into play, takes cognisance of the image, and conveys the idea to thebrain. With a little trouble, a real eye, say that of an ox, can be dissected out, and employed as a camera obscura, the operator seeing in the back of the eye, or “retina,” the same image which the ox would have seen if it had been alive.
In photography, the operator, when he has found that a perfect image is thrown upon the ground glass, which represents the retina of the eye, substitutes for it a sensitive surface, on which the rays are projected, and which, by chemical means, produce a permanent instead of a fleeting object.
Examplesof other lenses may be found in Nature. She, moreover, can perform a task which man has never even attempted, namely, the change of form in a lens according to the duty which it has to do. How this wonderful object is attained we shall presently see.
There is a form of lens extremely useful in Optics, namely, the “Plano-convex” lens. This is, in fact, one half of a double convex lens, the section being made through its edges, and the plane sides polished as well as the convex. As, however, this is only a half of the double convex lens, it does not need further explanation. Its natural counterpart may be seen in the annexed illustration.