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DRY PLATES, FILMS AND PAPERS 169
light rays for photographic purposes has been brought forward by Prof. R. W. Wood.1
It is a well-known fact that a landscape takes on quite a new aspect when viewed through tinted glass, as only a relatively small range of light rays from the objects are enabled to reach the eye through the glass. Some objects which do not reflect rays of such wave length as to be transmitted through the glass will therefore appear quite dark, while others which reflect such rays will appear much brighter by contrast. It seems quite probable that the sensitivity of the human eye to certain spectrum colours has become so developed because we are thus enabled to perceive the greatest amount of contrast between objects commonly observed.
Prof. Wood's experiments are interesting, since they show us in one instance, how our surroundings would appear if the retina of our eye was sensitive to the invisible ultra-violet rays, and in another, if it was sensitive to the infra-red rays.
In order to do this, care has to be taken that only those invisible rays from one end of the spectrum should be allowed to act upon the photographic plate during its exposure.
The screen used for photography with ultra-violet rays must, of course, be opaque to all visible light, and yet able to transmit ultra-violet light of wave lengths 3000-3200.
Glass of course is out of the question, since this is opaque to these rays.
Quartz lenses are usually employed when working with ultra-violet rays, but quartz is transparent to ordinary light.
To overcome this difficulty, Prof. Wood deposited a thin film of metallic silver on the quartz lens. This renders the lens quite opaque to ordinary light, but allows
1 Photo Journal, Oct. 1910, and Journal Astronomical Society, Jan. 1U10.