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396 THE PHOTOGRAPHIC NEWS. [June 22, 1883. same, but that the image on B is twice as large in every direction as it is on A, and that the brightness of the image on A must be four times that on B; which brings us a rule which applies equally well to lenses, and should there fore be remembered, viz., that the bi-iyhtness of an image varies inversely as the square of the distance of the aperture (or lens) from the focussing screen. This is an important point to bear in mind in photographic optics. There is yet another point to call to mind, which is, that the illumination of an image varies according, as it is opposite the aperture or at one side. Fig. 3. Suppose we have two points of light, A and A', of equal brightness, A being exactly opposite the aperture a h, and A' at the same distance from it, but on one side. In the first case, the point of light would show a light circle on the screen I; in the latter the shape would be nearly the same, but the hole in the card would practically be diminished, because the light would have to pass through it askew, the available aperture in the direction shown being d e, which is smaller than a b, and the same quantity of rays would not reach the screen. The diminution is evi dently considerable if the light be very aslant to the aper ture. Again, let us suppose that the aperture in the card were movable, so as always to present the same size of opening, and that the image is received on a flat screen. We have then to compare the image taken aslant as A' with that from A. The image I' will be bigger than I, because it is further away from the aperture, and the lightness of the image would on this account also be diminished. We shall see later that this applies not only to pinhole images, but to images formed by lenses, and has a practi cal bearing on the brightness of image at the centre and margins of a photographic plate, more especially when a wide angle lens is employed. Refection of Light.—When a beam of light falls on a surface, part of it is sent back; part being absorbed in all cases, and part may be transmitted. The less light absorbed by the substance, the smoother its surface; and the more opaque it is, the more perfect is it as a reflector. Polished silver reflects better than any substance with which we are acquainted, since the light reflected is colour less, and it will take a high degree of polish. If we take other metals which appear coloured, such as gold or copper, they naturally reflect less light, since the surface absorbs the rays which are complementary to that colour. This will be more perfectly understood further on. Glass reflects light least when the beam is perpendicular to the surface, and as the angle of incidence (that is, the angle at which it falls measured from the perpendicular to the surface) increases, the amount reflected is greater, and therefore less is transmitted. This points to the fact that in lenses the reflecting surfaces should be as few as possible, and the rays of light should fall on those surfaces as direct as possible. Experiment has show that when a beam of light falls upon a reflecting surface, the angle of incidence is equal to the angle of reflection. Figs. 4 and 5 will explain this. Fig. 4 shows the reflec ¬ tion from a flat surface, and fig. 5 that from a sphercial sur face. Risa ray of light falling (the incident ray) on these surfaces, and R‘ the reflected ray. The angles a and a, which the incident and reflected rays make with the per pendicular to the surface where the light stikes, are then always equal. Suppose light coming in one direction falls on an irregu lar surface as shown in fig. 6, it will be seen that all the rays will not be reflected in one direction, but will what is called scatter the light after reflection. The light reflected will, however, be brightest in the direction it would take if the surface were smooth. Thus white paper, ground-glass, and dirty or dewy lenses scatter light irregularly; and it is evident that the rays penetrating through a dirty lens will also be irregular in direction. If a clean lens gives a sharp image, it is evident that the dirty one cannot do so. This, perhaps, will be better understood when lenses are treated of. NO FILTERING FOR THE PRINTING BATH. BY R. H. CHAPMAN. Now, do not think me like the printing-bath when I express myself surprised in finding that all the presciibed treatments are encumbered with that tedious and somewhat wasteful pro cess—filtering. What is more tiresome than to daily filter a printing-bath, especially when you have once learned that it is wholly unnecessary ? Why some of the contributors on this subject failed to make mention of the following seems very strange, as it is old and has been in use by many photographers of this city for years. Make your bath the desired strength, then add some pulverized alum, say two or three drachms to a half-gallon bath, acidify slightly, shake thoroughly, let stand over night, and by morning it will have settled entirely clear, the alum having carried down all organic matter. Decant carefully, and now it is ready for