The Dictionary Of Photography

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

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Focal Length
telescope is now focussed on the moon, and, the focus temporarily fixed, the ground-glass is removed from the frame, and a negative of a strong contrasted subject is placed in the frame, from which the ground-glass has just been removed. A very good negative,
if handy, is that of a line-drawing. The eye-piece of the tele-scope is now placed in the same position as the card previously used - i.e., opposite the lens of the camera; a candle or lamp is placed behind the negative, and the camera racked-in till the
lines of the negative are seen sharp and clear. Now, as the telescope focus has been fixed for infinity, the negative will only appear sharp when it is at the true focus of the lens; this position is then accurately marked, as shown in fig. 65, in which t is the
Fig. 66.
telescope focussed for infinity, P the lens, and d h the position of the plane of the negative. Fig. 66 is, as will be at once seen, a combination of the two previous diagrams - figs. 64 and 65 - and the distance between the plane 1 and plane 11 is the true equivalent focus of the lens designated in the diagram by F.