The Optical Magic Lantern Journal (August 1895)

122 The Optical Magic Lantern Journal and Photographic Enlarger. Tue Photographic Salon wiil be held at the Dudley Gallery, Piccadilly, from September 30th to November 2nd. The usual charge of one shilling will be made to the public. * * * On page 108 of our number for June, in a letter from Mr. Jex Bardwell, in which he spoke of offering his services at the United States Photographic Convention, we headed it ‘“‘ Convention at Montreal,’’ instead of ‘‘Convention at Detroit.” Mr. Bardwell’s address is 938, Cass Street, Detroit, Mich., U.S.A. * * * THE annual exhibition of the Royal Photographic Society will be opened at the gallery of the Royal Society of Painters in Water Colours, Pall Mall, on September 30th. The secretary at 12, Hanover Square, W., will supply all information that may be required. * * * At the late tobacco exhibition at the Agricultural hall, Mr. W. D. Welford organised a photographic department, and did it well too, he having got together an excellent collection of pictures from various parts of the world. * * 4 On page XV. will be found a statement from the Autotype Co., in reply to the letter in our last with reference to the firm of Messrs. Elliott & Son, of Barnet. — ‘0: — Principle of Condensers, Spherical Aberration, etc. STAGE ILLUMINATION. A CORRESPONDENT writes:—‘‘I have been endeavouring to throw an ordinary limelight through coloured glass on to a small tableau, but I have not succeeded in getting the proper condenser. ‘The difficulty seems to be the very short distance between the apparatus and the object to be lighted—only some nine or ten feet—whilst the area that has to be covered is some seven feet at the least. Can you assist me as to how to do it ?”” This is a subject dealing with the manner in which rays of light falling on a condenser leave the same after passing through it, and, when the details are grasped , | by the reader, no difficulty ~~ -~ should be in the way of «.7s e gaining the desired illumination on any surface. --° --° For stage illumination, a single condenser, with what is known in the trade as a ghost-box, is employed. Given any particular plano-convex lens, which is termed a condenser, the focus of same may be ascertained by measuring at what distance from the lens parallel rays (approximately) come to a point. This may be determined by holding it convex side outwards towards, say, a window, on the wall of the room opposite which a piece of note paper has been affixed. In front of this, the lens is moved to and fro, until the image is as sharply depicted as possible under the circumstances. The measurement from the paper to the lens will give the focus. Supposing we start with a lens of the style mentioned, which is six inches in diameter with a focus of eight inches, this being a commercial size ; after fastening it in a board or box for the purpose, with a circular opening, we shall find that if we place the illuminant at a distance of eight inches from the lens (which must have its flat side next the illuminant) the projected rays will be practically parallel, and that if the light be brought further away the projected rays will come to a point, more or less, at some particular distance bearing a relation to that from which the light is distant from the condenser. The points of the source of light and that at which the projected rays meet are termed the conjugate foci. To make the meaning clearer, the following diagrams will assist. In Fig. I. the light a is placed at the focus of the lens, and it will be seen that the light is projected in a parallel manner, In Fig. II. it is placed sixteen inches off, and the projected rays come to a point about the same distance in front of the lens. |