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586 THE PHOTOGRAPHIC NEWS. [August 21, 1891. put off for days, weeks, or months, to suit the conven ience of the operator. Photography had, by the intro duction of these dry plates, suddenly found itself in a state of revolution, and the new took the place of the old with astonishing rapidity. But while the general public were busying them selves with the wonderful pictures obtainable by aid of the gelatine plate, the astronomer was not slow to see that this plate was the very thing that he had been waiting for so long. It answered all his requirements in responding to very feeble gleams of light, in remain ing sensitive for any time required during exposure, and keeping well after exposure and before develop ment ; for the pictures which he dreamed of taking, wheth er they were of planets, distant stars, or nebulae would require an exposure reckoned by hours rather than by seconds. The new agent was found to be fully equal to the call made upon it, and for the first time star charts were printed by their own feeble light. But the dry plate is something more than a mere registrar of stars; it is, in a manner, a discoverer as well. Who does not remember the story of the dis covery of the nebula attached to Maia, one of the stars in the Pleiades, and how it was found first of all in a photograph, its existence not having before been sus pected? Then we have the totally unlooked-for result that the dry plate registers stars so distant that they have never yet been seen by human eye, even when aided by the most powerful telescope ; for the action of the plate is cumulative, and these distant bodies, giving forth a light too feeble to excite the retina of the eye, by mere persistence of shining im press the sensitive plate. But while taking credit to photography for so much, should we not say a word for the wonderful precision of the instruments employed ? Who among us has not, in an idle moment, looked at Jupiter through the telescope of some itinerant astronomer, and has seen how the bright globe of light and its attendant satellites are plainly delineated on the dark blue of the sky? But, as we gaze, they are fast travelling out of the field of the telescope, and its owner has to give the instrument a jerk to one side in order that the image of the distant planet may once more run its course over the limited space. This motion, as we all know, is not due to Jupiter and his moons, but is an illusion caused by the movement of the earth itself. In our observa tories this movement is counteracted by a clock, which controls the telescope and keeps the objects in the same places in the field of view. How beautifully made and delicate this apparatus must be when it is found possible to keep the image in exactly the same place on a sensitive plate for several hours. Nay, it is possible to do more than this. The same plate can be exposed for several evenings in succession, the work broken off by clouds or other obstructions on one night being taken up on succeeding nights. This power of executing the portraiture of the stars by instalments is one of the many benefits brought to astronomical research by the modern photographic dry plate. PHOTOGRAPHY IN FRANCE. BY LEON VIDAL. French Photographic Society—Meeting of August 7, 1891—Retouching Negatives from the Back— Apparatus for the Measurement of the Field of Vision and of the Focal Length of the Lens— Apparatus for Preparing Concentrated Solutions —Trial of Sensitive Plates—Celluloid Dishes— The “Mignon” Instantaneous Shutter—Oil for Lubricating Shutters—Self-Stretched Films with a Metallic Border—Oxy-hydrogen Lamp—The Ether Light—Photography in Colours. At the French Photographic Society’s meeting on August 7th the attendance was but meagre, for the excursion season is at its height, and many members are at this time of year sojourning in the country. The process of retouching suggested by M. Poex seems difficult to realise in its normal conditions. He proposes to print a preliminary proof on paper, and to attach this by gum to the glass side of the negative. It would take too long to fully discuss this method, the weak points of which are immediately apparent. The method brought forward by M. Sebert is based upon a plan which has already been indicated by Monck- hoven. Its object is to measure the field of vision and the focal lengths of lenses. Two lines are traced on the focussing screen of the camera at a certain distance from one another, and the same distant point is alternately focussed. The same gentleman brought forward a method by which concentrated solutions can be conveniently prepared. Several manufacturers of dry plates were invited to send in specimen packages of their plates. M. Gravier stated that these plates had been submitted to trial in the presence of a few members of the Society. A small strip was cut from each of the different brands of plates, and these were placed side by side beneath a graduated screen, such as that employed in Warnerke’s sensitometer. The whole were then exposed to the light of a taper for a determined time, and afterwards developed. The scale of tints thus obtained readily showed the relative sensitiveness of the plates. From the firm of Block, some celluloid dishes for developing purposes, as transparent as those of glass, were brought forward by M. Gravier.* A new instantaneous shutter called the “Mignon” has been introduced, which has the merits of being cheap, easily manipulated, and capable of being used for instan taneous or time exposures. This shutter is small and compact, and is made entirely of metal. It is always set ready for action, and its mechanism is of such a character that it cannot readily get out of order. The firm of Dehors and Deslandres present some printing frames in hardened cardboard. They are light, and not so easily broken as those made of wood. The same firm also bring forward an oil made specially for lubricating shutters and other apparatus having moving parts. An automatically stretched film is brought forward by M. Planchon. In this case the emulsion is run upon a thin metallic frame which serves to keep the film, when dry, perfectly flat, and this permits it to be placed in the dark slide without any additional support. After deve lopment the frame is cut away and discarded, its work having been done. It remains to be seen whether this method will have any practical value. * These dishes are sold in London by Mr. Gotz, of Buckingham Street.