Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . double-backs with vulcanite are easily cleaned, and if they are rubbed with a piece ofwashleather they be-come charged with elec-tricity, and remove anydust that may be on thesurface of the plate whenthey are pulled advantage liesin the fact that they canbe pulled entirely out ofthe double-back. If aspring closes the slit inthe double - back, thelight is completely ex-cluded. Beginners are apt topay insufficient atten-tion to th
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . double-backs with vulcanite are easily cleaned, and if they are rubbed with a piece ofwashleather they be-come charged with elec-tricity, and remove anydust that may be on thesurface of the plate whenthey are pulled advantage liesin the fact that they canbe pulled entirely out ofthe double-back. If aspring closes the slit inthe double - back, thelight is completely ex-cluded. Beginners are apt topay insufficient atten-tion to the dust whichcollects on the plate andlens, and interferes withthe sharpness of theimage. It may becomea serious matter in a balloon; fine particles of sand from theballast sacks float all over the basket, and have a habit ofpenetrating everywhere, even through the tightest joints. Plates. Usually everyone settles for himself the plates to be used, andhas his own likes and dislikes. Novelties seldom find favour;they are regarded at first with suspicion, and only after manytrials do they cease to be novelties, and become trusted Fig. 195.—Aiguille Verte, taken with anordinary lens by the Vega Company, ofGeneva. 312 AIKSHIPS PAST AND PBESENT. But in balloon work, certain plates must be used if good resultsare to be obtained, though doubtless there is a certain latitudeallowable. Films are light and convenient, but the reasons for preferringglass plates have already been explained. Films are seldomquite flat, and it is therefore impossible to get a perfectly sharpnegative in consequence. The bigger the film the more unevenits surface is likely to be; even the most modern devices do notentirely remedy the defect. For the smaller sizes of negativeup to quarter-plate size, flat films in special carriers may beused. They are packed in black paper, and are placed in aspecial carrier against a glass plate, the paper being then pulled
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