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170 THE ADVANCE OF PHOTOGRAPHY
sufficient ultra-violet to pass through for the purpose of experiment.
It is much easier to arrange a suitable screen for the infra-red. A sheet of very dense cobalt glass which transmits light of wave-lengths 7100-7600 is employed, but since this allows some green and blue rays to pass through, it has to be supplemented by a cell containing a solution of bichromate of potash. By this means, rays of shorter wave length are cut out.
In the photographs of landscapes, when only ultraviolet rays are allowed to act upon the plate, the sky appears brighter than a sheet of white paper arranged so as to appear on the same photograph. On the other hand, when infra-red rays are used, the sky appears darker than the paper.
A very curious feature about the ultra-violet photographs is that no shadows appear on the developed plates, even when the exposure has been made in full sunlight.
In infra-red photographs the sky appears darker than the foliage and the shadows are quite black. With reference to the sky, it is noticed in Wood's photographs, that in the region of the zenith the sky appears quite black, but that it gradually becomes lighter towards the horizon. This is interesting as affording a visible illustration of the difference in the scattering of light produced by our atmosphere at varying altitudes above the horizon. On account of these effects the general appearance of a landscape containing dense masses of foliage is very similar to that obtained by ordinary photography when the trees are loaded with snow.
Dr Kenneth Mees x has pointed out that every small patch of cloud is distinctly visible in these infra red photographs, and has suggested that on this account they may prove useful in meteorological observations.
Chinese white, when photographed by ultra-violet rays,
1 Knowledge, Jan. 1911.