SEEING IN THE DARK.

Shannon News, 4 January 1927, Page 3

SEEING IN THE DARK.

i INVISIBLE LIGHT RAYS. There appeared recently in one of our newspapers a short account of the discovery by Baird of an invisible ray which might be expected to do wonderful things. In the absence of a more reliable scientific account of the discovery one hesitates to prognosticate, but it is unfortunately characteristic of sensational journalism that emphasis is laid upon "possibilities related to the uses of war (writes "Proton" in the Lyttelton Times). Baird, as we know, has been carrying out some important researches in connection with wireless television. Some account of his work was recently given when it was pointed out that his transmitter made use of ultra-violet light to modulate the carrier wave. It is pro bable that the new "ray" is simply a beam of ultra-violet light. The field of invisible light, waves is a fascinating one for the scientific explorer, and many wdnderful things have already been dene in tho way of using dark rays. Wireless itself, for example, might belaid to be essentially a' method of communication by means of invisible light waves. Ether waves may' be classified in much the same way sound waves, viz., in octaves, but, whereas the ear is sensitive to several actaves of sound, the normal eye perceives only one octave of light. As there are about fifty-three octaves of ether waves known to science the limitations' of direct vision will be appreciated. In immediate proximity to the visible octave —or what Ave call colour. —there, are the ultra-violet rays on the one hand and the infra-red rays on the other. Both of these bands, though dark to the human eye, are fairly wellknown and can be utilised for a number of purposes. It is well-known, for example, that ordinary photography utilises the ultraviolet. A photographic impression on a negative is largely tlie work of invisible light, which partially accounts for the fact that the tone values in a photograph of coloured objects differ so remarkably from the impression formed through drcct vkon. It may be news to some that it ia possible to take photos ia at room that appears to the eye to be absolutely dark. All that is necessary is to use a source of "illumination" rich in ultra-violet rays, to enclose it-in a screen of "chance" glass so as to allow only the ultra-violet rays to pass. It would lo necessary also to substitute for the glass lens of the camera one made of quartz, since ordinary glass is fairly opaque to ultra-violet. Suitable sources of illumination would be a carbon arc, a tungsten arc, a mercury vapour lamp or a calcium flare.

Astronomers make frequent uao of the ultra-violet in photographing the stars, and, '.n particular, the sun. in this case, however, the result is aecomplshed by wetting the camera in different parts of the invisible spectrum. In-fra-red photographs may also be taken with special plates. Direct vision of an object Humiliated by a beam of invisible light is of course impossible, but there are several indirect means of attaining Mie same object. It is possible, for example to change the wave length, lengthening it or shortening it so that it comes ultimately in the region of the visible spectrum. Tim is the well-known principle of fluorescence. If a strong beam of ultra-violet light is caused to play upon an object, an image of the object may be thrown, by means of a quartz lens, upon a fluorescent and so rendered visible. . Suitable fluorescent substances are willemite (silicate of zinc), barium platinocyanide, and uranium glass. A corresponding shift of infrared rays into the visible spectrum is called calorescence. For this purpose platinised platinum may be used. During the war vaiious applications of the above principles were utilised for secret signalling. A second and more complicated method, but one which extends the distance of vision enormously, has been made possible by the recent researches i\ television. A travelling spot of ultraviolet light may be used to modulate .1 carrier wave. This is subsequently demodulated by a neen lamp and the image thrown on a screen. This method involves, as will be seen, an ultimate change of wave length to the visible spectrum. It will be obvious that there is plenb of room for research in this field. Doubtless Baird's n-3\v discovery, if indeed it is new, is a further and more effective application of the above principles. Let us hope that it will be used for something better than a new weapon of war.