for the polarization of light is a very difficult subject; but this was the first step made in it. Fresnel afterwards worked out accurately why, when light is reflected at a certain angle, the vibrations are all made to move in one plane, and so the light is polarized, as Malus had found it to be from the surface of the Luxembourg windows. He also showed how in some crystals, as in quartz crystals, the waves are made to act upon each other, so that, instead of moving to and fro, they wind round and round like the wire of a corkscrew. These and many other experiments, as for example, those upon the beautiful colours caused by polarization, were carried much farther by the eminent French chemist, M. Biot (born 1774, died 1862), and by Sir David Brewster (born 1784, died 1868), but they are too long and difficult to be explained here. As I said at the beginning of this chapter, the Theory of Light' requires a special study, and if you have understood something of the movement of the supposed ether waves-how they can interfere with each other and produce light or darkness, how they produce coloured rings in the soap-bubble, and how their vibrations are altered in passing through a crystal or in reflection at certain angles-you have learnt as much as can be easily grasped of the discoveries of Young and Fresnel.

Chief Works consulted. -Young's 'Lectures on Natural Philosophy,' 1845; Peacock's 'Life and Works of Young;' Arago's Éloge of Fresnel;' Herschel's 'Lectures on Familiar Subjects;' Tyndall, 'On Light;' Spottiswoode's 'Polarization of Light ;' Whewell's Inductive Sciences; Encyclopædia Britannica '-Sixth Dissertation.

SCIENCE OF THE NINETEENTH CENTURY (CONTINUed). History of Spectrum Analysis-Discovery of Heat-rays by Sir W. Herschel-And of Chemical Rays by Ritter of Jena-Photography first suggested by Davy and Wedgwood-Carried out by Daguerre and Talbot-Dark Lines in the Spectrum first observed by Wollaston -Mapped by Fraunhofer-Life of Fraunhofer-He discovers that the Dark Lines are different in Sun-light and Star-light-Experiments on the Spectrum of different Flames--Four new Metals discovered by Spectrum Analysis--Bunsen and Kirchhoff explain the Dark Lines in the Solar Spectrum-Metals in the Atmosphere of the Sun-Huggins and Miller examine the Stars and Nebulæ by Spectrum Analysis.

History of Spectrum Analysis, 1800-1861.-We now come to the history of Spectrum analysis, or the study of the various coloured bands produced by different kinds of light when seen through a prism. This is certainly one of the most wonderful discoveries of our century, and though its history is difficult, partly because it belongs to our own time and is going on even now, yet we may learn something about it. The first step was made, as you will remember, when Newton discovered that white light is composed of different coloured rays, but even he little suspected what histories those rays could be made to tell.

Discovery of Heat-rays by Sir William Herschel, 1800. -One of the first facts which was learnt in this century about the spectrum was, that the coloured band which is seen when a ray of white light is passed through a prism does not

give us the whole of the dispersed ray; for there are many invisible rays at both ends of the coloured part which are very active, though we cannot see them.

It had always been thought that the hottest rays must be those, such as the yellow ones, which give the most light, and in the year 1800 Sir William Herschel, wishing to try this, took a thermometer and passed it gradually from one end to the other of the coloured band. The result was curious. He began at the violet end of the spectrum (Plate I. No. 1, p. 320), and, as he expected, the thermometer rose higher and higher as he approached the yellow part; but to his surprise it did not stop here. When he passed on through the yellow into the red, the heat still increased, and even became more intense as he passed out of the coloured band altogether into the darkness beyond. By this experiment he found that the heat-rays extend for some distance beyond the red colour, and that they are strongest in that part where no light is to be seen.

Discovery of Chemical Rays by Ritter, 1801.-Soon after Sir William Herschel had discovered the dark heatrays, a still more remarkable fact was brought to light about the violet end of the spectrum. The Danish chemist Scheele, who you will remember as one of the discoverers of oxygen (see p. 232), had once remarked that nitrate of silver will turn black if the violet rays of a spectrum are thrown upon it. In 1801, Professor Ritter, of Jena, was repeating this experiment, and he found that the black patches appeared slightly on those parts of the paper where the violet rays fell, but very strongly indeed beyond those rays where the spectrum was quite dark. So that at this end also there are invisible rays, and these have the extraordinary power of decomposing or breaking up nitrate of silver, and some other

CH. XXXIII.

PHOTOGRAPHY.

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substances, so as to leave distinct marks upon anything touched by them.

Photography.-You will see at once that this is the secret of Photography. In 1802, Sir Humphry Davy and Dr. Thomas Wedgwood suggested that pictures might be taken in this way by the rays of the sun acting upon chloride of silver, and they even succeeded in making some. But they could not prevent them from fading away again, and it was not until 1839 that a Frenchman named Daguerre learnt how to fix the pictures so that they would remain, and Mr. Fox Talbot afterwards improved the process. We cannot enter here into a complete account of photography, but you may form some idea of how the rays of light produce a picture.

When you go to have your photograph taken, the glass plate which is to receive your picture has been bathed in nitrate of silver, with some other chemicals. When you stand in front of it and it is uncovered, a luminous image of your face or body, which has been brought to a focus on the lens of the camera, falls upon the plate, and the chemical rays (which are chiefly those beyond the violet end of the spectrum) decompose the nitrate of silver. You can see nothing when the plate is taken out of the box in which it was placed, but by pouring some more chemicals called protosulphite of iron and pyrogallic acid upon it, the parts which the light has touched all start out in different shades, exactly in proportion as the chemical waves of light have. fallen upon it strongly or feebly. It will be exactly the opposite to your real appearance, because where most light has fallen, there the chemicals will be most decomposed and will leave the blackest tints.

Another fluid called hyposulphite of sodium is now

poured upon the plate to melt away any nitrate of silver which remains, so that when the sun next falls upon it it may not blacken the rest of the plate and destroy the picture. Then the glass plate is again placed in the sun with properly prepared paper under it; and now the shades are reversed. Under the dark parts of the plate the sun will act feebly on the paper, and produce light patches, while through the light parts it will act strongly and produce shadows. And in this way the lights and shades of your image will appear in their right places on the paper. All this work is done by the chemical rays which are chiefly at and beyond the violet end of the spectrum, and this explains why bright red and yellow objects come out dark in a photograph, because these colours contain so few chemical rays, while the darkest blue and violet come out nearly white, because they act strongly upon the nitrate of silver.

Wollaston first observes the Dark Lines in the Spectrum, 1802. In the same year that Ritter discovered the chemical rays at the dark end of the spectrum which have given us the whole art of photography, Dr. Wollaston, one of our most celebrated chemists (born 1766, died 1828), first saw the dark lines in the spectrum which have enabled us to discover the actual materials which exist in the sun and stars. Dr. Wollaston, who made many good experiments on light, was one day examining ordinary daylight through a prism, and instead of letting in the light by a round hole in the shutter as Newton had done, he made only a very thin slit, so that the colours of the spectrum were prevented from overlapping each other, as they had done in Newton's experiment. The result was that seven dark upright lines or spaces appeared in the band of colour, which seemed to show that no light fell on those parts. Wollaston did nothing more