Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . ographic lensis to be employed, or whether a simple lens with long focallength is sufficient. The following explanations must be givento clear up the matter. Working with a simple lens the image of distant objects is ata distance behind the lens equal to the focal length. Thereforethe ratio of the size of the image to that of the object is thesame as that of the focal length to the distance of the object. IfB is the size of the object, b that of the imag
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . ographic lensis to be employed, or whether a simple lens with long focallength is sufficient. The following explanations must be givento clear up the matter. Working with a simple lens the image of distant objects is ata distance behind the lens equal to the focal length. Thereforethe ratio of the size of the image to that of the object is thesame as that of the focal length to the distance of the object. IfB is the size of the object, b that of the image, E the distance of the object, and / is the focal length, then b = —^- • There-in fore if the distance is 100 times the focal length, the size of the object will be 100 times that of the image. In order to get large images of distant objects, it is therefore necessary to use a lens of great focal length. It is often suggested that it would be sufficient to take a small negative, and enlarge it in the usual way. But this is only possible within certain limits. Probably an enlargement which is five times the size of the negative is. Fig. 198.—Mont Blanc, as seen from Geneva ; ta I &Zf F$a;jj0«***s
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