and the hypotheses propounded by M. Berard, in 1812, which were reported upon by Berthollet, Chaptal, and Biot.* A few years passed away: Professor Schönbein discovered guncotton, and at the meeting of the British Association at Southampton, in 1846, he introduced it as an important improvement upon gunpowder. As a destructive agent, gun-cotton has been slow in making its way as an agent for projecting cannon-balls or for rending rocks; but dissolved in ether, it forms that collodion which Mr. Archer, in 1851, taught us how to use in multiplying images of the beautiful, and the process to which it has given its name is now universally adopted, to the almost entire exclusion of every other kind of photographic manipulation. At a very early period (1838-40), it was seen that the changes produced on the salts of silver by the sun's rays might be used to render meteorological and other instruments self-registering. In 1838, Mr. T. B. Jordan, then secretary of the Royal Cornwall Polytechnic Society, devised and used photographic methods for registering barometers, thermometers, and magnetometers. These methods, modified by Mr. Brooks and Mr. Ronalds, were subsequently introduced into the observatories at Kew and at Greenwich, where, at the latter especially, under the direction of Professor Airy, a beam of artificial light now registers through each day and night every movement of those steel bars which tell us of the variations in the earth's magnetic intensity, and of the occurrence of the strange phenomena known as "Magnetic Storms," now proved, by the investigations of General Sabine, to be intimately connected with those solar spots which are being explored-if the term is admissible-by Celestial Photography. While photography was making progress as an art, it was employed in a few hands as an aid in scientific investigations. Sir John Herschel especially used chemical compounds, sensitive to solar influences, to determine the relative values of the solar radiations proceeding from different parts of the sun's disc, and this led to the determination of the * "To show clearly the great disproportion which exists in this respect between the energies of different rays, M. Berard concentrated, by means of a lens, all that part of the spectrum which extends from the green to the extreme violet, and he concentrated, by means of another lens, all that portion which extends from the green to the extremity of the red. This last pencil formed a point so brilliant that the eyes were scarcely able to endure it, yet the muriate of silver remained exposed more than two hours to this brilliant point of light without undergoing any sensible alteration. On the other hand, when exposed to the other pencil, which was much less bright and less hot, it was blackened in less than six minutes.' -"Report of the Commissioners:" Annales de Chemie.' See also," Report on the Chemical Action of Solar Radiations: "Transactions of British Association for 1850,' vol. lxxxv., p. 309. "On a New Method of Registering the Indications of Meteorological Instruments." By T. B. Jordan. 'Sixth Report of Royal Cornwall Polytechnic Society,' 1838. 2 B VOL. III. fact at the same time, by two distinct observers, that the chemical action produced by the rays coming from the edge of the sun were less active than those proceeding from its central regions. This fact has been, strangely enough, recently put forward as a discovery by Professor Roscoe,† without the mention of any previous observer, excepting Secchi, whose observations had reference to the calorific, and not to the chemical radiations. It is true that Professor Roscoe has made a series of excellent experimental observations, and that he has proved "that the intensity of the chemically active rays at the centre is from three to five times as great as that at the edge of the disc;" but in doing this he has only confirmed the results already published. For example, in 1840, Sir John Herschel, in the Philosophic Transactions' (Part I., p. 43), distinctly stated that he had detected "a real difference between the chemical agencies of those rays which issue from the central portion of the sun's disc, and those which, emanating from its borders, have undergone the absorptive action of a much greater depth of its atmosphere, and yet I confess myself somewhat at a loss what other cause to assign for it. It must suffice, however, to have thrown out the hint, remarking only that I have other, and, I am disposed to think, decisive evidence of the existence of an absorptive solar atmosphere extending beyond the luminous one. The breadth of the border, I should observe, is small, not exceeding 0.5, or one-seventh part of the sun's radius; and this, from the circumstance of the experiment, must necessarily err in excess." Mr. Robert Hunt, in the Philosophical Magazine' already quoted, noticed the same phenomenon, and gave the same, as being the most familiar explanation of it; and subsequently M. Arago, in his 'Memoirs on Photometry,' again drew attention to this important fact. The results which have been obtained since 1840 appear to show, not merely that the chemical radiations generated near the edge of the solar disc are absorbed in passing through a greater depth of the sun's atmosphere, but that there is an actual interference (using this term in its ordinary acceptation rather than its scientific sense) exerted by the luminous radiations, and that the chemical radiations have their origin in a lower zone, that which produces Light-energy. The protected band which is seen to surround the prismatic image of the sun is not due to a lowering * Philosophical Magazine, vol. xvi., 3rd scries, contains an abstract of the memoir read before the Royal Society by Sir John Herschel; and also a paper in the same monthly part of this magazine by Robert Hunt, on "Experiments and Observations on Light which has permeated coloured Media, and on the Chemical Action of the Solar Spectrum," in both of which this fact was, for the first time, stated. "On the Measurement of the Chemical Brightness of various Portions of the Sun's Dic." By Henry Enfield Roscoe, B.A., F.R.S. Received June 12, 1863. Sce "On the Present State of our Knowledge of the Chemical Action of the Solar Radiations." A report to the British Association, in 1850, by Robert Hunt. merely of chemical (actinic) intensity, as would be the case if it were an instance of loss by the absorption in the solar atmosphere; but there is evidence of a changed condition, such as is shown by the protected bands observed under the yellow and the red rays, where luminous and calorific power attain their maximum influence. On this point we have yet a few more words to say. Professor Bond, of Cambridge, with Messrs. Whipple and Black, of Boston, in the United States, were the first to make a photographic picture of any celestial body. This was an image of the moon, obtained upon a Daguerreotype plate, which had been placed in the focus of the refracting telescope of the Harvard Observatory. In 1851, some of these Daguerreotypes of our satellite were in the American department of the Great Exhibition. In 1852, Mr. Warren De la Rue obtained positive lunar photographs, in from ten to thirty seconds, on a collodion film, by means of an equatorially-mounted reflecting telescope of thirteen-inch aperture and ten-feet focal length. At this time Mr. De la Rue had not applied any mechanical driving motion to his telescope. He was therefore constrained to contrive some other means of following the moon's apparent motion. This he accomplished by hand in the first instance, by keeping a lunar crater always on the wire of the finder, by means of the ordinary hand-gear of the telescope, but subsequently by means of a sliding frame fixed on the eye-piece holder, the motion of the slide being adjustable to suit the apparent motion of the moon. As the pictorial image of the moon could be seen through the collodion film, and could be rendered immovable in relation to the collodion plate, by causing one of the craters to remain always in apparent contact with a broad wire, placed in the focus of a compound microscope affixed to the back of a little camera-box which held the plate, this was effective. Excellent results were obtained under the disadvantages of the want of an automatic driving motion, which proved how perfectly the hand may be made to obey the eye. Mr. Warren De la Rue was admirably aided in his earliest experiments by Mr. Thornthwaite, since it was found impossible to work without the assistance of an experienced coadjutor. In 1853, Professor John Phillips communicated to the Hull meeting of the British Association the results of his experience in Lunar Photography, and he then exhibited some excellent pictures of our satellite. Mr. Hartnup, of Liverpool, aided by Mr. Crooke and other photographers, took some good pictures of the moon in 1854. Father Secchi, at Rome, Mr. Fry, in Mr. Howell's observatory at Brighton, and Mr. Huggins, now so well known by his application of spectrum analysis to the stars, nebulæ, and comets, also produced lunar pictures. A great extension of Celestial Photo |