Volltext Seite (XML)
[August 10, 1883. THE PHOTOGRAPHIC NEWS out apertures as in figs. 23 and 24. The cards themselves must be large enough entirely to cover the lens when placed against it. The card cut as in fig 23, will allow the Fig. 23. Fig. 24. central portion of the lens alone to be open, whilst, when an aperture in a card like fig. 24 is used, the margins of the lens will be uncovered. Focus with the card fig. 23 in contact with the lens, and then change it for that repre sented in fig. 24. It will be found that to obtain sharpness in the two cases, the focus will be altered (unless a lens happens to be used which, by the refractive index of the glass, is virtually free from aberration). This will show that, as a rule, spherical surfaces will give aberration of this description. There are special cases where there is no appreciable spherical aberration. Thus, a glass which has a refractive index of 1686 (or nearly that), if made into a plano-convex lens having the anterior surface convex, the margin and the centre of the lens will bring parallel rays to focus at the same point. Such being the case, if a point of light be placed at this focus, the rays emerging will be parallel. This form is used for condensing lenses when the glass has a high refractive index. Again, with crown glass having a refractive index of 1-5, if the radii of the spheres into which they are formed be as 6 to 1, the spherical aberra tion will be a minimum. For glasses of other refractive index, calculations can also be made to give a minimum. 8 Hictionary fit Ahotography. fight-screen proposed in the Photographic News, 1881, p. 38. This consists of a scale built up of various thicknesses of sulphur. See Sensitometer. Warnerke’s actinometer consists of a metal box, in which is a Fig. 4. resistance of selenium diminishes when exposed to light. A specially arranged cell of crystalline selenium and a galvano meter are included in an electric circuit, when every variation of the light shining on the selenium affects the galvanometer needle. Many sunshine-recording instruments based on the actinometers of Jordan, Herschell, and Hunt, have been introduced from time to time, but we need only refer to two of the most recent and convenient sunshine recorders. Mr. Campbell’s apparatus is thus described in the Photographic News :— “ About thirty years ago, Mr. Campbell, of Kensington, fitted a ball of glass into a wooden bowl, the adjustment being so made as to ensure the coincidence of the heat focus of the solar rays with the surface of the wood, so that whenever the instrument is exposed to sunshine of sufficient intensity, the great luminary leaves its trace in the shape of a charred mark on the internal surface of the wooden bowl. This somewhat crude apparatus was found to yield most valuable results, as the depth to which the Fig. 2. circular opening covering a disc painted with Balmain’s luminous paint, and when the box is closed an eye-piece ranges opposite to this painted disc, this arrangement being shown by figs. 2 and 3. After exposing the phos phorescent surface to the light, and before closing the instrument a graduated disc (fig. 4) is interposed between the eye-piece and phospho rescent surface ; and as this disc is turned round, increas ingly opaque parts come be tween the eye-piece and the luminous surface; the last letter or figure which can be read on the graduated screen „ serving to indicate the inten- 419: 6 ' sity of the light to which the luminous paint was exposed. Vidal has devised a somewhat complex but very efficient actino meter, in which advantage is taken of the fact that the electrical It is by no means necessary to describe in detail the various in struments which have been devised for enabling the photographer to judge of the actinic intensity of the light, especially as such instruments are seldom employed during actual work. Johnson’s actinometer is, however, of considerable value to the carbon printer, and it consists of a small metal box containing a roll of sensitive albumenized paper so arranged that it can be drawn under an opening in the lid of the box, the space around this opening being so coloured as to resemble silver paper which has been exposed to light for a short time. When the exposed part of the sensitive paper matches the painted portion of the lid, one tint is said to be registered, and the actinic intensity of the light is inversely as the time required for the registra tion of one tint. Woodbury’s actinometer is a somewhat similar instrument, but is pro vided with a graduated scale of tints round the opening. Actinometers in which stepped screens built up out of varying thicknesses of paper or other translucent material, and having a num ber impressed on each grade, are occasionally employed, the screen being placed over a sensitive surface and ex posed to light. Under these circumstances, the last num ber which is visible on the sensitive surface indicates the intensity of the light. Warnerke’s sensitometer consists of a graduated screen of this character, and by noting the effect when a standard exposure is given, it becomes practicable to estimate the relative sensitiveness of various plates. All light-screens or graduated actinometers hitherto proposed are quite arbitrary, and conse quently incapable of exact reproduction, excepting the sulphur