air, and the other from light passing through an equal length of water—were found to be such as to prove that the velocity of light in air and in water is as 4 to 3, while by the Newtonian theory it is as 3 to 4. By this final and decisive proof the undulatory theory may from henceforth be regarded as the law of nature. This experiment was accomplished by M. Fizeau and M. Léon-Faucault, at the suggestion of M. Arago, whose eyesight did not permit him to undertake it himself. SECTION XXIV. Chemical or Photographic Rays of Solar Spectrum—Scheele, Ritter, and Wollaston’s Discoveries—Wedgwood’s and Sir Humphry Davy’s Photographic Pictures—The Calotype—The Daguerreotype—The Chromatype—The Cyanotype—Collodion—Sir John Herschel’s Discoveries in the Chemical Spectrum—M. Becquerel’s Discoveries of Inactive Lines in ditto—Thermic Spectrum—Phosphoric Spectrum—Electrical Properties—Parathermic Rays—Moser and Hunt’s Experiments—General Structure and antagonist Properties of Solar Spectrum—Defracted Spectrum. THE Solar Spectrum exercises an energetic action on matter, producing the most wonderful and mysterious changes on the organised and unorganised creation. All bodies are probably affected by light, but it acts with greatest energy on such as are of weak chemical affinity, imparting properties to them which they did not possess before. Collodion and metallic salts, especially those of silver, whose molecules are held together by an unstable equilibrium, are of all bodies the most susceptible of its influence; the effects, however, vary with the substances employed, and with the different rays of the solar spectrum, the chemical properties of which are by no means alike. As early as 1772 M. Scheele showed that the pure white colour of chloride of silver was rapidly darkened by the blue rays of the solar spectrum, while the red rays had no effect upon it: and in 1801 M. Ritter discovered that invisible rays beyond the violet extremity have the property of blackening argentine salts, that this property diminishes towards the less refrangible part of the spectrum, and that the red rays have an opposite quality, that of restoring the blackened salt of silver to its original purity; from which he inferred that the most refrangible extremity of the spectrum has an oxygenising power, and the other that of deoxygenating. Dr. Wollaston found that gum guaiacum acquires a green colour in the violet and blue rays, and resumes its original tint in the red. No attempt had been made to trace natural objects by means of light reflected from them, till Mr. Wedgwood, together with Sir Humphry Davy, took up the subject: they produced profiles and tracings of objects on surfaces prepared with nitrate and chloride of silver, but they did not succeed in rendering their pictures permanent. This difficulty was overcome in 1814 by M. Niepcé, who produced a permanent picture of surrounding