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 with curved legs. These are termed Callipers, and can be used on a plane as well as on a rounded surface.
Natural Callipers 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 on [page 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 is Sialis armata, and it is a native of Columbia.
OPTICS.
CHAPTER I.
THE MISSIONS OF HISTORY.—THE CAMERA OBSCURA.—LONG AND SHORT SIGHT.—STEREOSCOPE AND PSEUDOSCOPE.—MULTIPLYING-GLASSES.
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.