lopes, red, orange, yellow, etc., corresponding to each of the bands of the spectrum. The nearer the sun is to the horizon the larger is the visible part of the rainbow; but, as the sun rises, the arc diminishes, and entirely disappears when the sun is 42 degrees above the horizon. Hence the rainbow is never seen except morning and evening.

CHAPTER VI.

INJURIOUS EFFECTS OF COLOUR IN LENSES.

ACHROMATISM.

346. Aberration of refrangibility.—In speaking of lenses we have been quite silent about a grave objection to which they are liable, which is, that at a certain distance objects seen through these lenses seem surrounded by an iridescent fringe, which fatigues the sight and greatly injures the precision of the images.

For, as lenses may be compared to a series of prisms with infinitely small faces, and united at their bases, they not only refract light, but also decompose it like a prism. On account of this dispersion, therefore, lenses have really a distinct focus for each colour. In condensing lenses, for example, the red rays, which are the least refrangible, form their focus at a point r on the axis of the

points.

Hence a double convex lens tends to give seven images unequally coloured of images seen through them. These images being partly superposed, the seven colours combine in the centre to form white light, but, on the contours, the extreme colours of the spectrum are visible, that is, more especially red and blue.

This injurious colouration of the images is called the chromatic aberration.

347. Achromatic lenses.-By observing the phenomenon of the dispersion of colours in prisms of water, of oil of turpentine, and of

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Spherical Aberration.

345

crown-glass, Newton was led to suppose that dispersion was proportional to refraction. He concluded that there could be no fraction without dispersion, and, there

fore, that achromatism was impossible. Almost half a century elapsed before this was found to be incorrect. Hall, an English philosopher, in 1733, was the first

Fig. 267.

re

to construct achromatic lenses, but he did not publish his discovery. It is to Dollond, an optician in London, that we owe the greatest improvement which has been made in optical instruments. In 1757 he combined two lenses, one a double convex crown-glass lens, the other a double concave lens of flint-glass (fig. 267), a kind of glass which contains a good deal of lead, and which has greater dispersive power than flint-glass.

By suitably choosing the curvatures of these two lenses, they may become unequally dispersive, and as the dispersion is in opposite directions, one of the lenses being convergent and the other divergent, two effects are produced, which compensate each other as regards colouration, but not as concerns refraction; that is, a ray of white light which has traversed such a lens emerges colourless, but converging, and forming a single focus on the axis.

The lenses thus formed of flint and crown-glass give images which are not coloured on the edges; they have hence been called achromatic lenses; achromatism being the term applied to the phenomenon of the refraction of light without decomposition.

348. Spherical aberration.-Chromatic aberration is not the only defect which lenses present: they have another, which is known as spherical aberration, and which arises from the fact, that, apart from dispersion, the rays which traverse a condensing lens do not exactly coincide in a single focus. Those which traverse the lens near the edges are too much refracted as compared with those which traverse the central part; hence the former rays converge nearer to the lens than do the latter, in consequence of which the images are distorted.

This inconvenience is obviated in optical instruments by intercepting the rays which traverse the lens near the edge by diaphragms or stops, perforated by circular holes, which only allow the central rays to pass.

CHAPTER VII.

OPTICAL INSTRUMENTS.

349. The different kinds of optical instruments.-By the term optical instrument is meant any combination of lenses, or of lenses and mirrors. By their means the limits of vision have been enormously increased, and the most favourable influence has been exerted on the progress of science, by opening out new worlds to investigation which would otherwise have remained unknown. Optical instruments may be divided into three classes according to the ends they are intended to answer, viz. :-i. Microscopes, which are designed to obtain a magnified image of any object whose real dimensions are too small to admit of its being seen distinctly by the naked eye. ii. Telescopes, by which very distant objects, whether celestial or terrestrial, may be observed. iii. Instruments designed to project on a screen a magnified or diminished image of any object which can thereby be either depicted or rendered visible to a crowd of spectators; such as the camera lucida, the camera obscura, photographic apparatus, the magic lantern, the solar microscope, the photo-electric microscope, etc. The two former classes yield virtual images, the last, with the exception of the camera lucida, yield real images.

General composition of optical instruments. Of the various instruments enumerated above those of the first two groups consist essentially of two lenses: one called the object glass, or objective, receives the light from the objects, and concentrates it in a focus where it gives a small image; the other, called the eyepiece, or ́ocular, acts as a magnifying glass, is near the eye, and serves to view the image formed by the object glass. In what are called reflecting telescopes, a concave mirror is used instead of an object glass. Generally speaking the object-glass and the eyepiece are not formed of a single glass, but of several, in order to obtain a greater magnifying power, and to correct chromatic and spherical aberration. These glasses are, moreover, mounted in long metal tubes, blackened on the inside so as to absorb the oblique rays, which would otherwise injure the sharpness of the image; these tubes can, moreover, be slided in or out, so that the glasses may be brought to the proper distance.

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Galileo's Telescope.

347

350. Galileo's telescope.-Like many grand discoveries, that of the telescope seems due to chance. For it is stated to have been made accidentally by the children of a Dutch spectacle maker at Middlebourg. Looking at a vane on the top of a church spire through a convex and concave glass, the latter being nearer the eye, they were surprised to see the object magnified, and apparently almost within reach. The father repeated the experiment, and arranged the two glasses in tubes, one which slided in the other, and thus constructed the telescope.

This telescope bears Galileo's name, for this illustrious astronomer was the first to direct it towards the heavens, and to make astronomical observations. It is stated that he was at Venice when he learnt that Zacharia Jans had offered to Prince Maurice of Nassau an instrument which brought objects nearer : he quickly

Fig. 268.

started for Padua, where, after meditating on the matter, he made some experiments, and in twenty-four hours rediscovered the telescope.

The telescopes constructed by Galileo were gradually improved from a magnifying power of four up to one of thirty times. By its means Galileo discovered the mountains of the moon, Jupiter's satellites, and the spots on the sun. From these numerous discoveries he acquired the name Lynceus, from one of the Argonauts, whose sight is said to have been so penetrating that he could see to the bottom of the sea.

Fig. 268 represents the arrangement of the lenses and the path of the rays in a Galileo's telescope. The object glass, O, is a double convex, while the eyepiece, o, is a double concave lens. If AB is the object observed, the rays, from any one of its points, A for instance, tend to form an image of this point beyond the object-glass; but meeting the double concave lens, o, these rays appear divergent, and seem to the eye which receives them as if they proceeded from

the point a; and it is there the image of A appears. In like manner the image of B is formed at b, so that a virtual image, ab, is formed, which is erect, and very near.

Galileo's telescope is very short and portable. It has the advantage of showing objects in their right position, and, further, as it

has only two lenses, it absorbs very little light in consequence, however, of the divergence of the emergent rays it has only a small field of view, and in using it the eye must be placed very near the eyepiece. The eyepiece can be moved to or from the object