Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . singdifferent focallengths; the onlynecessary matter isthat the distancebetween the lensesshould not differfrom the sum of their focal lengths by an amount equal to thefocal length of the back lens. The advantage of this arrangement is evident. The length ofthe camera can be considerably shortened, whereas with simplelenses of great focal length the camera must be of a correspond-ing size. It has been already stated that a focal length of 24 the mo
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . singdifferent focallengths; the onlynecessary matter isthat the distancebetween the lensesshould not differfrom the sum of their focal lengths by an amount equal to thefocal length of the back lens. The advantage of this arrangement is evident. The length ofthe camera can be considerably shortened, whereas with simplelenses of great focal length the camera must be of a correspond-ing size. It has been already stated that a focal length of 24 the most that is possible for an amateur. It may now be well to consider why telephotographic lensesare not employed under all circumstances. The reason is thatthe image is not so sharp, and the intensity of the light whichfalls on the plate is reduced. If two lenses, each of a focallength of 8 in., are placed one behind the other, and the secondlens magnifies the image five times, then the image is as large asif it were given by a lens of 40 inches focal length, but its bright-ness is twenty-five times less. Therefore these combinations can. Fig. 197.—Diagram showing the relation betweenthe focal length of the lens, the size of the image,and the distance of the object. 318 AIRSHIPS PAST AND PBESENT. only be used in moderately clear weather ; in dull weather theybecome useless, because under such conditions and with suchlenses instantaneous photography would be impossible. Major Houdaille has stated that in his opinion telephotographiclenses are of no use in a balloon, but Baron von Bassus is notaltogether of this opinion, thinking that they may do muchuseful work for military purposes. There is a good deal to besaid for this latter view, but, as things at present stand, the Fig. 200.—Pyramids of Cheops, Chephren, and Mencheres.(Photograph by Spelterini.) amateur will probably save himself some disappointment if heuses the simple lens. We must now consider the conditions attachin
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