Marion and Co., compiler · London : Marion and Co. · leaf 111
a lens of double the focal length of the whole. (See Chapter on Optics.) We now come to the process of exactly focussing the view. We use the lens's full aperture. That is to say, we put no stop into the diaphragm slit, or if the dia- phragms be rotary we turn the diaphragm disc so that the largest… … Now we have to select the diaphragm to use. We shall probably perceive that, although the principal object is now quite sharp, neither the distance nor the foreground is so. We place the stop next largest to the full aperture in position, when we shall perceive a notable improvement on the…
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