590 Principal Focus of a Lens. it, the little image of the sun is most clearly defined, we can at once ascertain the focus or focal length of a glass. It is remarkable that the refractive power of the common glass used for lenses is such, that the focus of a double convex lens is just as distant from it as the centre of the sphere, of which its surface is a portion. This gives another fact with which to associate the recollection that the focus is nearer as the convexity of the lens is greater, that is to say, as the surface is a portion of a maller sphere. It may also be proved both by calculation and experiment that if a lens be held at twice its principal focal distance from a candle-suppose at c for a lens with the focus at a (fig. 196)— the image of the candle will be formed at / just as far on the other side. Thus, then, by trying with a lens until the image of a candle is formed at the same distance from it as that at which the candle stands, we have a second mode of ascertaining the focal distance of a lens, or the degree of its convexity. Other kinds of glass and other substances refract with different power; but the facts now stated should be retained in the memory as standards of comparison. Because the focal point of light passing through a lens is at the same distance from the centre of the lens, in whatever direction the light passes through, a surface placed to receive the picture of a broad field should really be concave, that is to say, all parts of it should be nearly at the same distance from the centre of the lens, otherwise the image will be more perfect either at its middle than towards its edges, or the contrary—but it is not found necessary to at end to this in common practice, when the object and its image are not of great extent. 820. The size of an image formed behind a lens is always proportioned to the distance of the image from the lens, and the image is as much larger or smaller than the object, as it is farther from or nearer to the lens than the object is. This will be evident from Fig. 197. a considering the figure (197), in which c represents the place of a lens, and the lens, according to its power, will form an image of the cross, a b, in some situation, as at d, e, or g. Now wherever the image is formed, and by whatever lens, one end of it must be in contact with the line, a g, and the other Magnifying power of Lenses. 591 end with the line, bh; and as these lines cross each other at c, and widen regularly afterwards, a line joining them (and the image is such a line), must always be shorter the nearer it is to c, that is to say, shorter in proportion to the converging power or focal distance of the lens. The narrow luminous circle called the focus of a burning-glass, is really but the image or picture of the sun formed by that glass or lens. The intensity of the heat and of the light is of course in proportion as the image is smaller than the glass which forms it, and the nearer that the image is formed to the lens, or the more powerfully convergent that the lens is, the smaller will the image be. Mr. Parker's famous burning lens, which cost £700, and became the property of the Emperor of China, was three feet in diameter, and the diameter of the sun's image formed by it was one inch it concentrated the light and heat therefore about 1,300 times. To render the effect still more powerful, a smaller lens was placed behind the larger, further reducing the size of the focal image. Surprising effects were produced by this lens, in the melting of metals, inflaming of combustibles, &c. The size of burning lenses, formerly, was limited by the difficulty of obtaining the great masses of glass required to form them; but glasses have since been built up of many pieces suitably united together. Some large lenses have been made of water, that is, of water inclosed between capacious glasses, formed like watch-glasses. A common spherical goblet of water, or a vase for holding gold-fishes, has in some cases acted as a burning-glass, setting fire to window-curtains, near to which it had been left in the sunshine. 821. The nearer an object is brought to a lens, the more distant, and therefore the larger, will the image formed by it become; for, as the rays falling upon a lens are divergent in proportion to the nearness of the object, and therefore with the same power of lens, must meet farther behind (as seen in fig. 197), so the axes of the sets of rays, as the lines, c a and c b, will be separated farther before the rays meet, and will have made the image proportionally larger. If we suppose the cross, d, in the same diagram to be the object, its image would be a b. The sun is exactly as much larger than his image formed by a burning-glass, as he is more distant from it than the image. From these considerations it follows that, in a camera obscura, the screen should, for distant objects, be at the distance of its principal focus from the lens, and a little farther off than this for near 392 The Magic Lantern. objects. The lens is usually fixed in a sliding piece, which allows the distance from the screen to be adjusted to circumstances. If the representation be desired large, the lens must be of a long focus if small, the lens must be of a short focus. Again, when by the reversed use of the lens, a small object, as d, is to be magnified on a screen or in the air to such a size as a b, then the object must be placed but a little beyond the principal focus of the glass; if it be placed nearer, the pencils of rays from it would never be gathered to focal points at all, and no image would be formed at any distance. When, as supposed in the last sentence, a small object is placed very near a strong lens, and the image is thrown upon the wall of a dark room, perhaps a hundred times farther from the lens than the object is, the image is a greatly magnified representation of the object, viz., it is a hundred times longer and a hundred times broader, and therefore has ten thousand times as much surface as the object. If, in such an experiment, the object be illumined only in an ordinary degree, the light, being diffused or diluted in the same degree as the image is enlarged, is too faint to suffice for distinct vision. Hence, to attain fully in this manner the purpose of a microscope, a very strong light, concentrated by a suitable mirror or lens, must be directed upon the object. When the light of the sun is used in such a case, the complete apparatus is called the Solar microscope, and serves well to display the structure of many minute objects. When artificial light is used, as that of a lamp, the apparatus is called the lucernal microscope or Magic Lantern. 822. The Solar microscope was highly valued until the improvements in the construction of lenses were made, by which the dispersion of light, or the rainbow fringe, was prevented, and until the electric light could be substituted for that of the sun. With the table microscope, only one person at a time can see the wonder; but with the solar microscope, or with the photo-electric microscope, a whole company may enjoy the spectacle simultaneously. The well-known Magic Lantern consists of a powerful lens, with objects, highly illumined by artificial light, placed so near it that their images are formed far off, and are therefore proportionally larger. The objects are generally paintings made on thin plates of glass with transparent colours; and the plates are formed to slide in a groove behind the lens, and are hence called slides. The distance of the lens from. the object may be varied, and thus a cor The Eye as an Optical Instrument. 593 responding approach to or receding from the screen is allowed, which will vary the magnitude of the visible picture on the wall. A thin mist or smoke at night will sometimes reflect the images of a magic lantern so as to make them distinctly visible; and this is one method of summoning up spectres by a concealed lantern upon an artificially-produced smoke. Another method is by the use of dissolving views. In this case two lanterns are employed, and in the midst of the scene, bright and clear, displayed by the one, appears more or less hazy or spectral as desired, the figure of the other illuminated by a less bright light. An improved form of magic lantern has been constructed by Mr. Woodbury, under the name of Sciopticon.* There are two lenses, and these can be so adjusted as to cover a surface, ten feet square, with a perfect illumination by the use of a Kerosene lamp. It can also be used for dissolving views and other purposes. "The EVE itself is, in fact, but a small camera obscura.” 823. The account above given of the camera obscura describes closely also that most interesting object, the living eye itself—the great inlet of man's knowledge, and the window through which is perceived much of what passes in the mind within! We shall describe the eye and its actions, keeping present the idea of the camera obscura; and we shall find that the nature and uses of the various parts of the eye are declared by merely naming them. This paragraph should be perused while the reader can observe his own eye reflected in a mirror, or the eyes of friends near him. The human eye, then, is almost a perfect sphere of the size of a large walnut, having for its outer wall, C, a very tough membrane, called, from its hardness, the sclerotic coat, which is, in common language, the white of the eye; in the front of this there is a round opening or window, E B, named, because of its horny texture, the cornea. The chamber is lined with a finer membrane or web, the choroid (having relation to colour), which, to insure the internal darkness of the place, is covered with a black paint, the pigmentum nigrum. This lining is bordered at the edge of the round window by what may be called a folded drapery, the ciliary processes, hidden from without by being behind the curious contractile window-curtain, the iris (so named from its rainbow variety of colour in different persons), through the central opening of which, called * From σkia, a shade, and OTTIKOS, optical. 594 Structure of the Eye. the pupil, the light enters. Immediately behind the pupil is suspended, by attachments among the ciliary processes, the crystalline lens, D, a double convex, perfectly transparent body of considerable hardress, which so refracts the light passing through it from external objects, as to form perfect images of these objects, in the way already described, on the back wall of the eye, G, over which the innumerable filaments of the optic nerve, called the retina, are spread as a sensitive lining. The eye is maintained in its globular form by a watery liquid, which distends its external coverings, and which, in the space before the lens, or the anterior chamber of the eye, being perfectly clear, is called the aqueous humour, and in the remainder or larger posterior chamber, being inclosed in a pellucid spongy structure, so as to acquire somewhat of the appearance of melted glass, is called the vitreous humour. 824. The annexed figure represents a vertical section of an eye of an average size, so as to show the edges of the coats, &c. C is the outer or sclerotic coat (fig. 198); A is the transparent cornea, somewhat resembling a watch-glass: it is more bulging than the sclerotic, or forms a portion of a smaller sphere than the general eye-ball, so that while it may be truly called a bow-window, it, with the convex surface of its contained water, forms a powerful lens for acting to converge the pencils of entering light. At B, and similarly all round the edge of the cornea, is attached the window-curtain or iris, shown here edgeways, immersed in the aqueous humour, and extending inwards from above and below towards its central opening or pupil, through which the rays of light are passing to the lens. The iris has in its structure two sets of fibres, the circular and the radiating, which cross and act in opposition to each other, keeping the membrane flat and tense; when the circular fibres |