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DRY PLATES, FILMS AND PAPERS 171
appears quite black, and even slight traces of it, which are invisible to ordinary eyes, make their presence known in this way.
Prof. Wood has already obtained some peculiar photographs of the moon by the use of ultra-violet rays, and from the results obtained, he suggests that it may ultimately be possible to obtain by this means information as to the nature of the rocks comprising the moon's surface.
Working at / 8, the exposure required for a landscape in bright sunlight, when making use of infra-red rays, is ten minutes. With ultra-violet light the time varies according to the thickness of the silver film employed, and ranges from 3 or 4 seconds up to § minute. Of course the photograph of the full moon requires a much longer time — 2 minutes.
Lantern Plate. — One other type of plate deserves mention, although it is not, as a rule, exposed in a camera like the other varieties : this is the Lantern plate. These are made in one standard size and are very much less rapid than ordinary photographic plates.
The emulsion with which they are coated contains either some soluble chloride (sodium chloride being frequently used) and silver nitrate, or else, in addition to the above, ammonium bromide and dilute hydrochloric acid. The plates are usually exposed to artificial light while in register with the negative the positive of Avhich it is thus desired to produce. For this purpose they can be placed in an ordinary printing frame with the film against the film of the negative, and exposure then made to some lamp or gas-light of known candle power, the frame being held at a fixed distance from the light.
Full particulars as to exposure, etc., are usually issued by the makers with each batch of plates, the speed of the plates being stated in some well-known unit. E.g. The