[Ill.u.s.tration]
6. An _incident ray_ is that which comes from any luminous body to a reflecting surface; and that which is sent back from a reflecting surface, is called a _reflected ray_. The _angle of incidence_ is the angle which is formed by the incident ray with a perpendicular to the reflecting surface; and the _angle of reflection_ is the angle formed by the same perpendicular and the reflected ray.
7. When the light proceeding from every point of an object placed before a lens is collected in corresponding points behind it, a perfect image of the object is there produced. The following cut is given by way of ill.u.s.tration.
[Ill.u.s.tration]
8. The lens, _a_, may be supposed to be placed in the hole of a window-shutter of a darkened room, and the arrow at the right to be standing at some distance without. All the light reflected from the latter object towards the lens, pa.s.ses through it, and concentrates, within the room, in a focal point, at which, if a sheet of paper, or any other plane of a similar color, is placed, the image of the object will be seen upon it.
9. This phenomenon is called the _camera obscura_, or dark chamber, because it is necessary to darken the room to exhibit it. The image at the focal point within the room is in an inverted position. The reason why it is thrown in this manner will be readily understood by observing the direction of the reflected rays, as they pa.s.s from the object through the lens. In the camera obscura, it is customary to place a small mirror immediately behind the lens, so as to throw all the light which enters, downwards upon a whitened table, where the picture may be conveniently contemplated.
10. From the preceding explanation of the camera obscura, the theory of vision may be readily comprehended, since the eye itself is a perfect instrument of this kind. A careful examination of the following representation of the eye will render the similarity obvious. The eye is supposed to be cut through the middle, from above downwards.
[Ill.u.s.tration: _a a_, the _sclerotica_; _b b_, the _choroides_; _c c_, the _retina_; _d d_, the _cornea_; _e_, the _pupil_; _f f_, the _iris_; _g_, the _aqueous humor_; _h_, the _crystalline humor_; _i i_, the _vitreous humor_.]
11. The _sclerotica_ is a membranous coat, to which the muscles are attached which move the eye. The _cornea_ is united to the sclerotica around the circular opening of the latter, and is that convex part of the eye, which projects in advance of the rest of the organ. The s.p.a.ce between this and the crystalline lens is occupied by the aqueous humor and the iris. The _iris_ is united to the choroides, and it possesses the power of expanding and contracting, to admit a greater or less number of rays.
12. The _crystalline lens_ is a small body of a crystalline appearance and lenticular shape, whence its name. It is situated between the aqueous and vitreous humors, and consists of a membranous sack filled with a humor of a crystalline appearance. The _vitreous humor_ has been thus denominated on account of its resemblance to gla.s.s in a state of fusion. The _retina_ is a membrane which lines the whole cavity of the eye, and is formed chiefly, if not entirely, by the expansion of the optic nerve.
13. The rays of light which proceed from objects pa.s.s through the cornea, aqueous humor, crystalline lens, and vitreous humor, and fall upon the retina in a focal point, to which it is brought, chiefly by the influence of the cornea and the crystalline lens. The image, in an inverted position, is painted or thrown on the cornea, which perceives its presence, and conveys an impression of it to the brain, by means of the optic nerve.
14. _Optical instruments._--The art of constructing optical instruments is founded upon the anatomical structure, and physiological action of the eye, and on the laws of light. They are designed to increase the powers of the eye, or to remedy some defect in its structure. In the cursory view which we may give of a few of the many optical instruments which have been invented, we will begin with the _spectacles_, since they are the best known, and withal the most simple.
15. The _visual point_, or the distance at which small objects can be distinctly seen, varies in different individuals. As an average, it may be a.s.sumed at eight or nine inches from the eye. In some persons, it is much nearer, and in others, considerably more distant. The extreme, in the former case, const.i.tutes _myopy_, or _short-sightedness_, and, in the latter case, _presbyopy_, or _long-sightedness_.
16. _Myopy_ is chiefly caused by too great a convexity of the cornea and the crystalline lens, which causes the rays to converge to a focus, before they reach the retina. Objects are, therefore, indistinctly seen by myoptic persons, unless held very near the eye to throw the focus farther back. This defect may be palliated by the use of concave gla.s.ses, which render the rays proceeding from objects more divergent.
17. _Presbyopy_ is princ.i.p.ally caused by too little convexity of the cornea and crystalline lens, which throws the focal point of rays reflected from near objects, beyond the retina. This defect is experienced by most people, to a greater or less degree, after they have advanced beyond the fortieth year, and occasionally even by youth. A remedy, or, at least, a palliation, is found in the use of convex gla.s.ses, which render the rays more convergent, and enable the eye to refract them to a focus farther forward, at the proper point.
18. The opticians have their spectacles numbered, to suit different periods of life; but, as the short-sighted and long-sighted conditions exist in a thousand different degrees, each person should select for himself such as will enable him to read without effort at the usual distance.
19. The great obstacle to viewing small objects at the usual distance, arises from too great a divergence of the light reflected from them, which causes the rays to reach the retina before they have converged to a focus. This defect is remedied by convex lenses, which bring the visual point nearer to the eye, and consequently cause the rays to concentrate in a large focus upon the retina. The most powerful microscopic lenses are small globules of gla.s.s, which permit the eye to be brought very near to the object.
20. _Microscopes_ are either _single_ or _double_. In the former case, but one lens is used, and through this the object is viewed directly; but, in the latter case, two or more gla.s.ses are employed, through one of which a magnified image is thrown upon a reflecting surface, and this is viewed through the other gla.s.s, or gla.s.ses, as the real object is seen through a single microscope.
21. The _solar microscope_, on account of its great magnifying powers, is the most wonderful instrument of this kind. The principles of its construction are the same with those of the camera obscura. The difference consists chiefly in the minor circ.u.mstance of placing the object very near the lens, by which a magnified image is thrown at the focal point within the room.
22. In the case of the camera obscura, the objects are at a far greater distance from the gla.s.s on the outside than the images, at the focal point, on the inside. The comparatively great distance of the object, in this case, causes the image to be proportionably smaller.
In the solar microscope, a small mirror is used to receive the rays, and to reflect them directly upon the object.
23. The _magic lantern_ is an instrument used for magnifying paintings on gla.s.s, and for throwing their images upon a white surface in a darkened room. Its general construction is the same with that of the solar microscope; but, in the application, the light of a lamp is employed instead of that from the sun.
24. _Telescopes_ are employed for viewing objects which from their distances appear small, or are invisible to the naked eye. They are of two kinds, _refracting_ and _reflecting_. The former kind is a compound of the camera obscura and the single microscope. It consists of a tube, having at the further end a double convex lens, which concentrates the rays at a focal point within, where the image is viewed through a microscopic lens, placed at the other end.
25. In the construction of reflecting telescopes, concave mirrors, or specula, are combined with a double convex lens. A large mirror of this kind is so placed in the tube, that it receives the rays of light from objects, and reflects them upon another of a smaller size. From this they are thrown to a focal point, where the image is viewed through a double convex lens. The specula are made of speculum metal, which is a composition of certain proportions of copper and tin.
26. Many optical appearances are of such frequent recurrence, that they could not have escaped the earliest observers; nevertheless, ages appear to have elapsed, before any progress was made towards an explanation of them. Empedocles, a Greek philosopher, born at Agrigentum in Sicily, 460 years before Christ, is the first person on record who attempted to write systematically on light.
27. The subject was successively treated by several other philosophers; but the ancients never attained to a high degree of information upon it. We have reason to believe, however, that convex lenses were, in some cases, used as magnifiers, and as burning gla.s.ses, although the theory of their refractive power was not understood.
28. The magnifying power of gla.s.ses, and some other optical phenomena, were largely treated by Al Hazen, an Arabian philosopher, who flourished about the year 1100 of our era; and, in 1270, Vitellio, a Polander, published a treatise on optics, containing all that was valuable in Al Hazen"s work, digested in a better manner, and with more lucid explanations of various phenomena.
29. Roger Bacon, an English monk, who was born in 1214, and who lived to the age of seventy-eight, described very accurately the effects of convex and concave lenses, and demonstrated, by actual experiment, that a small segment of a gla.s.s globe would greatly a.s.sist the sight of old persons. Concerning the actual inventor of spectacles, however, we have no certain information; we only know that these useful instruments were generally known in Europe, about the beginning of the fourteenth century.
30. In the year 1575, Maurolicus, a teacher of mathematics, at Messina, published a treatise on optics, in which he demonstrated that the crystalline humor of the eye is a lens, which collects the rays of light from external objects, and throws them upon the retina. Having arrived at a knowledge of these facts, he was enabled to a.s.sign the reasons why some people were short-sighted, and others long-sighted.
31. John Baptista Porta, of Naples, was contemporary with Maurolicus.
He invented the camera obscura, and his experiments with this instrument convinced him, that light was a substance, and that its reception into the eye produced vision. These discoveries corresponded very nearly with those by Maurolicus, although neither of these philosophers had any knowledge of what the other had done. The importance of Porta"s discoveries will be evident, when it is observed, that, before his time, vision was supposed to be dependent on what were termed _visual rays_, proceeding from the eye.
32. The telescope was invented towards the latter end of the sixteenth century. Of this, as of many other valuable inventions, accident furnished the first hint. It is said, that the children of Zacharias Jansen, a spectacle-maker, of Middleburg in Holland, while playing with spectacle-gla.s.ses in their father"s shop, perceived that, when the gla.s.ses were held at a certain distance from each other, the dial of the clock appeared greatly magnified, but in an inverted position.
33. This incident suggested to their father the idea of adjusting two of these gla.s.ses on a board, so as to move them at pleasure. Two such gla.s.ses inclosed in a tube completed the invention of the simplest kind of the refracting telescope. Galileo greatly improved the telescope, and constructed one that magnified thirty-three times, and with this he made the astronomical discoveries which have immortalized his name.
34. John Kepler, a great mathematician and astronomer, who was born at Weir, in Wurtemburg, in the year 1571, paid great attention to the phenomena of light and vision. He was the first who demonstrated that the degree of refraction suffered by light in pa.s.sing through lenses, corresponds with the diameter of the circle of which the concavity or convexity is the portion of an arch. He very successfully pursued the discoveries of Maurolicus and Porta, and a.s.serted that the images of external objects were formed upon the optic nerve by the concentration of rays which proceed from them.
35. In 1625, the curious discovery of Scheiner was published, at Rome, which placed beyond doubt the fact, that vision depends upon the formation of the image of objects upon the retina. The fact was demonstrated by cutting away, at the back part, the two outside coats of the eye of an animal, and by presenting different objects before it. The images were distinctly seen painted on the naked retina.
36. Near the middle of the seventeenth century, the velocity of light was discovered by Roemer; and, in 1663, James Gregory, a celebrated Scotch mathematician, published the first proposal for a reflecting telescope. But, as he possessed no mechanical dexterity himself, and as he could find no workman capable of executing his designs, he never succeeded in carrying his conceptions into effect. This was reserved for Sir Isaac Newton; who, being remarkable for manual skill, executed two instruments of this kind, in the year 1672, on a plan, however, somewhat different from that proposed by Gregory.
37. In the course of the year 1666, the attention of Sir Isaac Newton was drawn to the phenomena of the refraction of light through the prism; and, having observed a certain surprising fact, he inst.i.tuted a variety of experiments, by which he was brought to the conclusion, that light was not a h.o.m.ogeneous substance, but that it is composed of particles, which are capable of different degrees of refrangibility.
38. By the same experiments, he also proved, that the rays or particles of light differ from each other in exhibiting different colors, some producing the color red, others that of yellow, blue, &c.
He applied his principles to the explanation of most of the phenomena of nature, where light and color are concerned; and almost every thing which we know upon these subjects, was laid open by his experiments.
39. The splendor of Sir Isaac Newton"s discoveries obscures, in some measure, the merits of earlier and subsequent philosophers; yet several interesting discoveries in regard to light and color, as well as many important improvements of optical instruments, have been made since his time, although the light by which these have been achieved, was derived princ.i.p.ally from his labors.
[Ill.u.s.tration: GOLDBEATER.]
THE GOLD-BEATER, AND THE JEWELLER.
GOLD.
1. The metals most extensively employed in the arts are gold, silver, copper, lead, tin, and iron. These are sometimes found uncombined with any other substance, or combined only with each other; in either of these cases, they are said to be in a _native state_. But they are more frequently found united with some substances which, in a great measure, disguise their metallic qualities, or, in other words, in a state of _ore_. The mode of separating the metals from their ores, will be noticed in connexion with some of the trades in which they are prepared for, or practically applied in, the arts.
2. Gold is a metal of a yellow color, a characteristic by which it is distinguished from all other simple metallic bodies. As a representative of property, it has been used from time immemorial; and, before coinage was invented, it pa.s.sed for money in its native state. In this form, gold is still current in some parts of Africa; and even in the Southern states of our own country, in the vicinity of the gold mines, the same practice, in a measure, prevails.
3. Gold is rarely employed in a state of perfect purity, but is generally used in combination with some other metal, which renders it harder, and consequently more capable of enduring the friction to which it is exposed. The metal used for this purpose is called an _alloy_, and generally consists of silver or copper.
4. For convenience in commerce, this precious metal is supposed to be divided into twenty-four equal parts, called _carats_. If perfectly pure, it is denominated gold 24 carats fine; if alloyed with one part of any other metal or mixture of metals, it is said to be 23 carats fine. The standard gold coin of the United States and Great Britain is 22 carats fine; or, in other words, it contains one-twelfth part of alloy. Gold, made standard by equal parts of copper and silver, approaches in color more nearly to pure gold than when alloyed in any other manner.
5. Gold is found in veins in mountains, most usually a.s.sociated with ores of silver, sulphurets of iron, copper, lead, and other metals. It is often so minutely distributed, that its presence is detected only by pounding and washing the ores in which it exists. But the greatest part of the gold in the possession of mankind, has been found in the form of grains and small detached ma.s.ses, amid the sands of rivers and in alluvial lands, where it had been deposited by means of water, which had detached it from its original position in the mountains.
6. To separate or extract gold from the foreign matters with which it may be combined, the whole is first pounded fine, and then washed by putting it in a stream of water, which carries off the stony particles, while the gold, by its specific gravity, sinks to the bottom. To render the separation still more perfect, this sediment is mixed with ten times its weight of quicksilver, and put into a leather bag, in which it is submitted to a pressure that forces the fluid part through its pores; while the more solid part of the amalgam, which contains most of the gold, remains.
7. To separate the quicksilver from the gold, the ma.s.s is subjected to the process of _sublimation_ in earthen retorts, which, as applied to metals, is similar in its effects to distillation, as applied to liquids. When gold is contained in the ores of other metals, they are roasted, in order to drive off the volatile parts, and to oxydize the other metals. The gold is then extracted by amalgamation, by liquefaction with lead, by the aid of nitric acid, or by other methods adapted to the nature of the ore.