Photographic optics and colour photography : including the camera, kinematograph, optical lantern, and the theory and practice of image formation / by George Lindsay Johnson.

  • Johnson, G. Lindsay (George Lindsay), 1853-1943.
Date:
1909
Catalogue details

Licence: In copyright

Credit: Photographic optics and colour photography : including the camera, kinematograph, optical lantern, and the theory and practice of image formation / by George Lindsay Johnson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    308/348
    large condenser (5| in.) and alum tank, which answers admirably for quarter-plate Lumiere colour slides. In practice it is found that in order to get the best results, the focal length of the condenser should be proportional to the equivalent focus of the objective. It is also essential that the brightest part of the flame should be accurately centred in the axis of the condenser, and furthermore, that the distance of the radiant from the condenser should be so adjusted (which can only be done by trial) that the coloured ring formed round the illuminated disc on the screen disappears. The disc should then be uniformly white and quite free from colour. To get this perfect requires some practice in adjusting and centering. If the light be above the principal axis of the condenser, the dark coloured semicircle, A, will be seen on the upper margin of the disc (Fig. 177). If below, the half-ring will be near the lower margin, B. Should the light be too near the condenser, a com- plete dark ring, with a bluish fringe, will be seen occupying the middle third of the disc C. If the light be too far away, the ring will have a bluish fringe and will surround the periphery of the disc D. If both centering and distance be correct, the disc on the screen will appear uniformly white. In order to increase the light, a convergent mirror is sometimes placed behind the luminant, which should be placed at its focus. § 103.—(3) The Projecting- System.—This usually con- sists of a 6-in. or 8-in. focus Petzval portrait lens. Perken FRONT Fig. 178.—Perken & Son's Lantern Lens. & Son have produced an excellent form at a very low price (Fig. 178), but one of the many forms of rectilinear anastigmats of large working aperture, F/4 to F/5, is to be preferred, as they have a natter field and, consequently, a better marginal definition,. Still the Petzval form performs well enough for any ordinary exhibition, and is much cheaper.