The Optical Magic Lantern Journal (October 1892)

112 InsreaD of the illuminant being fastened to the body of a triple or bi-unial lantern, Mr. H. C. Newton has worked out an idea for securing it to the lantern front. In this way, when the fronts containing the optical parts are tilted, the illuminant still remains in the optical axis of the system. * a a Invention describes a simple form of praxinoscope as follows :—‘‘ Take two negatives of the same person, but in two different positions ; for example, taking water at a pump. In the first picture the arm should be raised, then lowered in the second, and the two attitudes of the individual will be in relation with the two positions of the arm of the pump. From these negatives make prints, and placc the two images on the opposite surfaces of a piece of cardboard, the two images being reversed with reference to each other. Tie a string on each side of the cardboard, and, by twisting it between the fingers, you will cause the cardboard to revolve rapidly, and obtain the sensation of the motion of a person pumping water. * * * Mr. A. R. Dresser has taken about three hundred hand camera negatives of scenes in connection with Buffalo Bill’s entertainment, and intends to issue sets of lantern slides of same. a k >» New words are continually being applied to the photographic art-science. The latest is supplied by Messrs. R. & J. Beck in connection with their new hand camera, which is termed the Frena. The picture that is taken with it is a Frenagraph; the person taking it, the Frenographer, and the particular solution which they advocate for a developer, Frenole. * * * Tue Thornton-Pickard Manufacturing Co. (late St. Mary’s Street, Manchester) having just completed a fine factory and offices at Altrincham, near Manchester, have moved the whole of their business under one roof. The factory has been fitted with the best machinery and appliances for the manufacture of their wellknown shutters, cameras, lenses, &c. x * * A CoupLe of days before going to press, we attended the opening of the Exhibition of the Photographic Society of Great Britain, 5a, Pall Mall, and those who visit it cannot fail to be delighted. The hours are from 10 a.m. till 5 p.m. On Monday, Wednesday, and Saturday evenings, the Hall will be open from 7 to 10, when slides will be projected by the lantern. The Optical Magic Lantern Journal and Photographic Enlarger. Flatness of Field. Tis is a term which is understood by most lanternists, but as cur readers are of all classes— inexperienced as well as experienced—a few words to the former, descriptive of the term, may not be out of place. Roundness of field is discovered when the picture is projected upon the screen. The centre appears pertectly sharp, whilst the sides are indistinct, and if the lens be racked until the image at the margins is sharp, the central portion appears blurred and indistinct. In single lenses, rays entering at the margins come to a focus closer to the surface than those which pass through the lens nearer the centre, and, as a consequence, there are several focal points according to the different parts of the Image. Some of this indistinctness may be remedied by using a stop, or diaphragm, in conjunction with the lens, thus causing marginal rays to be cut off, the image then becomes shapened, but at the expense of light. The well known Petzval portrait lens is in general use for lantern work. Many lenses of this type give a somewhat round field, this may be remedied to some extent by a slight judicious alteration. Between the two lenses of the back Back. combination will be found a ring of metal, which serves to keep the parts from touching each other. In many cases flatness of field may be improved by reducing the thickness of the ring, thus causing the elements, of which the back combination is formed, to be brought closer together. The lenses are secured in cells, and these in turn are screwed into the brass tube which forms the mount. Some makers secure the lenses in the cells by a ring screw, and when finished in this style, it is a simple matter to get at the back combination so as to reduce the thickness of the separating ring spoken of. Other makers burnish the glasses in, by turning over the edges of the cells, and in this case, some experience is required to get the lenses out of their mounts. In the case of the burnished in lenses, it is well, before again fastening them permanently, to ascertain with accuracy that the lens has | been altered to its best capabilities,