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460 THE PHOTOGRAPHIC NEWS. [JULY 20, 1883. at D and O'. Cover up N C‘ with a screen of paper, and have a movable screen for N C with a hole pierced in it. Place the piece of paper in such a position that the image from ABC and A 1 B' C' coincide, and now move the movable screen along N C till the image formed by the portion of the prism BLMN also falls on the other images. Do the same with a movable screen for N C'. Then we shall have an image at O, but four times brighter than would be given by one prism. By increasing the number of prisms having slightly different angles, the brightness of the image might be perpetually increased. If, instead of a number of pieces of different prisms, a piece of glass be ground into two cylindrical surfaces, or a plain and cylindrical surface, noting that the middle part is the thickest, we may, instead of holes in our screen, use a continuous cut-out line, and we shall get the same result, for we may look upon such a piece of glass in section as composed of an indefinite Fiq. 14. number of such prisms (fig. 14). If, instead of cylindrical surfaces to the glass, we use spherical surfaces, we may abolish the screen altogether, for practically, and also by calculation, it is found that rays of light falling from one point of light on such spherical surfaces are brought together so as to meet very nearly in one point on the oppo site side of the lens, and this point is called the focus of the lens for that particular point of light of which an image is formed. Such a piece of glass is called a lens, and the special kind of lens we have had under consideration is called a bi-convex lens. With a spherical surface or surfaces, it is evident that brightness of the image will be proportional to the area of the glass struck by the rays of light emanating from a source. Fig. J&. If two prisms be placed point to point, and screened as before, it will be found that the rays of light will never cross, and that an intensified brightness can never be obtained. By increasing the number of prisms the same result would be found to hold good ; and finally, by using a glass ground into two hollow spherical surfaces, the like would still be found to hold good. This lens is a bi-con- cave lens. All lenses which are thinnest in the middle can have no real focus, though they do possess what is called a virtual focus, with which we need not trouble ourselves. Having seen how a lens is formed, it will be well to enumerate the different kinds of lenses. Fig. 16. I. is a bi-convex lens. II. is a plano-convex lens. III. is a concavo-convex or meniscus lens. IV. is a bi-concave lens. V. is a plano-concave lens. VI. is a convexo-concave lens. E ittionary fit 49hotography. ACHROMATIC.—(i, negative prefix: xpwfia, colour.)—A compound lens, constructed with the view of bringing so many of the rays to focus at one point as shall prevent the production of those coloured fringes which characterise images produced by uncorrected lenses. Perfect achromatism is a theoretical impos sibility ; but in actual practice it is generally sufficient to ensure the coincidence of two or three points in the spectrum. Su Lenses. ACIDS, Bases and Salts.—Groups into which one great class of chemical substances has been divided, the distinction being, however, so ill marked that it is sometimes difficult to know whether to regard a substance as an acid, a base, or a salt. Water may very well be regarded as the central figure of the whole series, it being possible, without any very great effort of the im agination, to regard the substance as an acid, a base, or a salt A so-called hydrogen acid consists of hydrogen united either with a halogen, or a compound group capable of acting like a halogen, 8 s hydrochloric acid, HC1; hydrobromic acid, HBr ; hydriodic acid, HI; or hydrocyanic acid, HCN. The salts corresponding these, and, indeed, to all of the acids, are derived by substituting a metal for the hydrogen. If, for instance, the hydrogen in HC1 is replaced by silver, we obtain the salt AgCl,or chloride of silver ; and if the hydrogen in (two molecules of) the same acid is replaced by zinc, we obtain ZnCl 2 , or chloride of zinc. The leading idea of a salt is a substance obtained by replacing the hydrogen of an acid by a metal; but as the term metal itself is somewhat vague, and as hydrogen itself may be regarded as the vapour of an exceedingly volatile metal, it is a common practice to speak of acids as hydrogen salts or salts of hydrogen. Acidity, in the popular sense of the term, is not by any means a universal quality of acids, although by far the greater part of those acids which are soluble in water are sour to the taste, and redden blue litmus paper. The second and principal great family of acids is that compris: ing the so-called oxygen acids; and these may be regarded 48 compounds of hydrogen united with oxygenated groups, whicn groups may have a certain analogy with the halogens; but in th majority of cases these oxygenated groups have not been isolated As characteristic examples of oxygen acids we may refer to HC,H,0,, acetic acid ; H,SO, sulphuric acid ; and H,PO,ortho phosphoric acid ; while corresponding salts would be NaCHa0» sodium acetate; K:SO,, normal potassium sulphate; KH5 hydrogen potassium sulphate ; CasO,, calcium sulphate, NasPO,, normal or tri-sodium phosphate; Na,HPO, di-sodium hydrogen phosphate (common phosphate of soda). . A base may be regarded as a metal, or group acting as a meto, united with the radicle HO ; examples being afforded by Kl > potassium hydrate, or caustic potash; (N#,) HO,, ammonite hydrate ; and Ca(HO)a, calcium hydrate, or slaked lime. Bas nd when soluble in water, have ordinarily an alkaline reaction, a restore the blue colour to litmus which has been reddened by The fundamental reactions between acids and alkalies consis