When dry, the artist may engrave into this coat of plaster of Paris, by means of a lithographic engraving needle, any design or picture with the greatest ease; the plate or glass is thereby laid bare, and design or picture appears black through the plaster of Paris coating. Mistakes or errors are easily remedied by filling in the plaster of Paris preparation.
With the regular printers’ roller composition a stereotype is now made of the picture or design on the glass or plate, in the usual way; some bichromate of ammonia solution should be added to the roller composition, to make the stereotype hard enough for the type press, and it will be as durable as any electrotype, and answer the same purpose.--Am. Lith. and Printer.
Stereoscopic Photography.--Of late, there is quite a revival in this branch of our art-science, several English and many foreign amateurs having been working with twin lenses during the last and present seasons. The Belgian Bulletin has an article on the subject, and the last technical meeting of the Photographic Society was devoted to it. Although Wheatstone announced the instrument in 1838, it was not until photography had come to his aid by furnishing satisfactory diagrams, and Brewster had popularized the matter by the invention of the lenticular stereoscope, that much progress was made; then Wheatstone gave his Bakerian lecture on January 15th, 1852, to put the finishing touch to this important branch of scientific work. The earlier attempts failed by reason of employing too wide an angle.
ECLIPSE PHOTOGRAPHY.
Probably in no department of science, certainly in no branch of astronomical science, has photography been of such use as in the study of solar eclipses. It is only when the sun is obscured by the moon that we are able to see and properly photograph the corona or luminous atmosphere around the sun. This solar corona, as has been said by Young, “is visible only about eight days in the century in the aggregate, and then only over narrow strips of the earth’s surface, and but from one to five minutes at a time by any one observer.” Very little of the eight days, however, can be utilized; indeed, as has been pointed out by Miss Clerke in her admirable History of Astronomy During the Nineteenth Century, the corona has only been observed by scientific men during forty-five minutes in as many years. Opportunities of observing an eclipse occur therefore at such comparatively long intervals, the phenomena to be observed are so varied and extensive, and the time during which the observations must be made is so very limited, that any permanent records of the phenomena, such as photography enables us to obtain, cannot fail to be of the greatest value. The most careful drawings of the same eclipse by different observers at the same station are so very dissimilar that it is generally unsafe to base any conclusion on them; whereas in photographs we have truthful records of the actual phenomena without personal equation of any kind, and with the additional advantage that there is more detail in the photograph than it is possible to insert in any drawing made during an eclipse, or even at leisure after the three or four minutes’ observation of such an indefinite and irregular object as the corona. The history of the increase of our knowledge of the corona is practically the history of the improvement of our photographic methods of attacking the phenomena of an eclipse.
The first occasion on which photography was used at an eclipse of the sun was on July 8, 1842, when Professor Majocchi, at Milan, attempted to obtain Daguerreotype pictures of the corona. His account of the attempt informs us that “a few minutes before and after totality an iodised plate was exposed in a camera to the light of the thin crescent, and a distinct image was obtained; but another plate exposed to the light of the corona for two minutes during totality did not show the slightest trace of photographic action. No photographic alteration was caused by the light of the corona condensed by a lens for two minutes, during totality, on a sheet of paper prepared with bromide of silver.” No details are given of the apertures of the lenses employed, or of their focal lengths. At the outset, therefore, astronomers were met with failure, but the failure at Milan did not deter Dr. A. H. Busch and Herr Berkowski from a similar attempt at Konigsberg on July 28, 1851. The telescope used on this occasion had an aperture of 2.4 inches, and a focal length of 30 inches. Commencing immediately after the beginning of totality, a plate was exposed for 84 seconds in the focus of the telescope, and on development an image of the corona was obtained. A second plate exposed for from 40 to 45 seconds was fogged by the sudden breaking out of the sunlight. The picture thus obtained--the first photograph of the corona and prominences--is known as the Konigsberg Daguerreotype, and is still preserved at the Strasburg Observatory. It was lent by Professor Winnecke for the exhibition of scientific instruments at South Kensington in 1876. On it the prominences, and the lower portion of the corona extending about one-fourth of a solar diameter from the moon’s limb, are distinctly shown, the encroaching of the prominences on the dark disc of the moon, owing to irradiation, being particularly evident.
Daguerreotype was again used for the annual eclipse of May 26, 1854, by Mr. Campbell and Professor Loomis at New York; by Dr. Bartlett and Victor Prevost, who obtained nineteen photographs, at West Point; and by Professor Stephen Alexander and Mr. E. H. Old at Ogdensburg.
Liais in 1858 obtained photographs of the partial phases, using wet plates. On one of these the moon can be seen projected on the corona before totality. With the introduction of the collodion process more sensitive plates were obtained, and a great advance was anticipated. At the total eclipse of 1860, July 18, Mr. Warren de la Rue, at Rivabellosa, in Spain, used wet plates. His instrument was one specially devised by himself for photographing the sun’s disc for sun-spots, and is known as the Kew heliograph. It is an ordinary equatorial mounting with driving clock, carrying a photographic object-glass, 3.4 inches clear aperture, and 50 inches focal length. The primary image is .466 of an inch in diameter, but before the image falls on the plate it is enlarged by an ordinary Huyghenian eyepiece to 3.8 inches diameter. The exposing apparatus for the ordinary sun photographs is an instantaneous shutter; this, of course, had to be abandoned for the eclipse photographs. Two plates were exposed during totality, the exposure being 60 seconds in each case, but only slight traces of the corona were obtained. At the same eclipse Father Secchi and Professor Monserat, working at Desierto de las Palmas, obtained good photographs of the corona, using an object-glass of .15 metre diameter, and 2.5 metres focus, the primary image being 23 millimetres in diameter. The plates were placed in the primary focus and according to Secchi, “all the phases of the phenomena are represented on the photographs.” The original negatives obtained at Desierto de las Palmas of this eclipse have unfortunately been lost.
The next attempt at photographing the corona was on August 18, 1868, this being remarkable as the first attempt to use a reflector for the purpose. Colonel Tennant and Sergeant Phillips at Guntoor used a 9–inch silver-on-glass mirror, by With, of 6 feet focal length, mounted equatorially by Browning on the Newtonian plan.