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. CALLIPERS. [Illustration: JAWS OF SIALIS.] [Illustration: 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 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.