124 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Drss. VI. 



(564.) 

 Chemical 

 rays of the 

 spectrum. 



(565.) 

 They in- 

 terfere ; 

 and may be 

 polarized. 



(566.) 

 Mrs So- 

 merville's 

 experi- 

 ments. 



violet rays (when the sun's light is decomposed by a 

 prism), in the same manner as the invisible rays of 

 heat were found by Sir Wm. Herschel to extend be- 

 yond the visible red. Scheele had indeed previously 

 noticed that the power of the sun's light to decom- 

 pose and blacken salts of silver increased rapidly 

 from the red towards the violet ray ; and there is little 

 doubt that Herschel's discovery suggested that of 

 Hitter, of the independent or non-luminous rays of 

 the spectrum. 



Bitter attributed to these rays a deoxidizing qua- 

 lity. Dr Wollaston, who also made experiments on 

 the subject, and discovered the specific action of the 

 different rays on gum-guiacum, prudently suggested 

 the more comprehensive term of chemical rays. Va- 

 rious other denominations have been proposed, which 

 need not be here dwelt upon ; all that can be said is, 

 that deoxidation does not represent the solar action 

 completely. Whether it be really an independent 

 principle in the sun's rays which causes these effects, 

 or merely light and its modifications, is, as in the 

 corresponding case of heat, yet undecided. But it 

 is remarkable that, as shown by M. E. Becquerel, 

 the discontinuity of the luminous spectrum produc- 

 ing " Fraunhofer's Lines" exists equally for the che- 

 mical rays. 



Soon after Bitter's first experiments, Dr Young 

 proved the Interference of the obscure chemical 

 rays (Phil. Trans., 1803), a conclusion successively 

 claimed since by different physicists. Berard in 1812 

 showed that these rays are polarized by reflection. 

 Seebeck observed that the different rays impressed 

 different colours upon salts of silver : and M. Edmond 

 Becquerel long afterwards showed that the red and 

 yellow rays, though incapable of commencing chemical 

 action, in some instances have the power of continu- 

 ing it when once excited by the more refrangible 

 rays. 



In 1835 Mrs Somerville made some interesting 

 experiments on the permeability of different bodies 

 to the chemical rays, similar to those of Melloni on 

 the heating rays (see chap. VI., 8), and she found 

 great and seemingly capricious variations in this re- 

 spect. 1 The account of them was addressed to Arago, 

 and published in the proceedings of the French Aca- 

 demy. She found that green glass, coloured by cop- 

 per, intercepted entirely the chemical rays, yet this was 

 not due so much to its colour as to its other qualities 

 (which are also peculiar as regards radiant heat), for 



the emerald transmits the same rays. Red glass 

 stops most of the chemical rays, whilst the garnet 

 transmits them. Though white glass has generally 

 been considered very transparent for these rays, Mr 

 Stokes has shown that it entirely stops those of the 

 very highest refrangibility, which are readily trans- 

 mitted by quartz. Sir John Herschel and Mr Hunt 

 have made many interesting experiments with coloured 

 media on the particular parts of the chemical spec- 

 trum absorbed, but a great deal remains to be done, 

 especially as regards the nature of the substances 

 employed. 



But the great impulse given to this subject was 

 derived from the invention or discovery of the beau- 

 tiful art of PHOTOGRAPHY. 



In 1802 Mr Thomas Wedgewood and Sir Hum- 

 phrey Davy succeeded in forming pictures of objects 

 laid on paper prepared with nitrate of silver, and in 

 taking profiles (silhouettes) by means of shadows. 

 They proposed to obtain similar effects by means of 

 the camera obscura, but their paper was not suffi- 

 ciently sensitive. The effectual bar to their proceed- 

 ings was, however, this : that they could discover no 

 means of fixing the shadows which they had ob- 

 tained, or preventing the whole surface of the paper 

 from being gradually blackened by exposure to light. 



In 1814 J. NICEPHORE NIEPCE, a retired proprie- 

 tor at Chalons sur Saone, 2 entered into a similar en- 

 quiry, but by methods quite different. He employed 

 the solar effect upon resinous bodies, and some 

 at least of his pictures were executed on plates of 

 pewter or of rolled silver. They were mostly copies 

 of engravings, and the light parts corresponded to the 

 lights of the originals. He, however, at length suc- 

 ceeded in fixing impressions of views in the camera 

 obscura, though in an imperfect manner, and after very 

 long exposure. The pictures thus obtained had this 

 in common with more perfect processes, that the lu- 

 minous impression was first brought into view by 

 a chemical process subsequent to exposure in the 

 camera. In 1825 Nicephore Niepce became associated 

 with Daguerre. who had previously been engaged in 

 the same research ; they agreed to communicate the 

 results of their several experiments. The result, as 

 is well known, was the invention of the DAGUERREO- 

 TYPE, not improperly called after Daguerre, who 

 seems really to have worked it out almost entirely 

 for himself, after the death of Niepce in 1833; 

 whilst so patient and determined was Daguerre in 



(567.) 

 History o1 

 photogra- 

 phy- 



(568.) 

 Wedge- 

 wood and 

 Davy. 



(569.) 

 Nicephore 

 Niepce. 



Daguerra 



Mrs Mary 1 The maiden name of this accomplished lady was Mary Somerville ; she was born, I believe, at Jedburgh, and married first Mr 

 Somerville Greig, afterwards her relative Dr Somerville. She is known in British science not only as the able commentator of Laplace's Meca- 

 magne- nique Celeste, but as the author of some ingenious and apparently convincing experiments on the magnetizing power of the violet 

 tic action ray. Some anomaly, however, remains to be explained on this subject, as the result cannot always be obtained. Several years 

 of light [?]. before Dr Faraday made his discovery of what he terms the "magnetization of light" (see chapter vii., 5), the writer of 

 these pages supposed that the reaction of light and magnetism, observed by Morichini and Mrs Somerville, might be due to a la- 

 tent and casual polarization of the light which was not present in all the experiments ; and, in particular, he suspected that circu- 

 larly polarized light might have a magnetic influence ; but his experiments to this effect, in May 1836, were not successful, 

 though he thinks them worth repeating. 



2 Probably one of the MM. Niepce who, in the early part of the century, are said to have propelled a boat on the Saone by a 

 peculiar kind of air-engine, called pyreolophore. Delambre, Rapport Historique, 1810, p. 242. 



