The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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ground of the paper. A long time (several weeks) is often required for this, but heat accelerates the action, and it is often complete in a few hours. In this state the picture is to be very thoroughly rinsed and soaked in pure warm water, and then dried. It is then to be well ironed with a smooth iron, heated so as barely not to injure the paper, placing it, for better security against scorching, between smooth clean papers. If, then, the process has been successful, a perfectly black positive picture is at once developed. At first it most commonly happens that the whole picture is sooty or dingy to such a degree that it is condemned as spoiled, but on keeping it between the leaves of a book, especially in a moist atmosphere, by extremely slow degrees this dinginess disappears ; and the picture disengages itself with continually increasing sharpness and clearness, and acquires the exact effect of copperplate engraving on a paper more or less tinted with pale yellow."
Amplifier, or Magnifier. A supplementary convex lens (ordinarily a spectacle lens), to be adjusted in front of the lens on a fixed focus camera when near objects are to be photo-graphed." The formulae bearing on the question are as follows : - Letl be the respective foci of the two positive elements
of the combination, and d the distance between them ; then
F is the focus of the combination. When the lenses are close together, so that distance apart is neglected, the formula becomes: -
If the added lens is a negative or dispersing lens of focal length /*, the formula becomes : -
or, neglecting the element of distance (lenses close together, or nearly so),
(See Lens ; also Adon.)