A beam of sunshine contains radiation having a wide range of wavelengths. The human eye is only sensitive to about one octave of wavelength, from 0.4 microns (0.0004 mm.) to o.8 microns (o.0008 mm.), corresponding to violet and red light respectively. Roughly one-half of the energy of a beam of sunshine is contained within the limits of visible light, and most of the remainder is accounted for by infra red or heat rays. Rays whose wavelengths are between 0.4 microns and about 0.014 microns are known as ultraviolet rays. Such rays are present in the solar beam, but with relatively small intensity. They are of little or no meteorological importance, so far as our present knowledge extends, but they are of very considerable importance in photography and in their actions on living organ isms of all kinds. These rays are readily absorbed or scattered by impurities in the atmosphere, and even by relatively clear air, and one of the most serious effects of the smoke of great cities is the removal of the ultraviolet rays from the solar beam. Ordinary window glass also absorbs completely the ultraviolet rays of wavelength below 0.31 microns. Special types of window glass are however obtainable which transmit fairly freely ultra violet light down to 0.2 microns. Ordinary sunlight contains no rays of wavelengths below about 0.29 microns.
Sir Leonard Hill states (Sunshine and Open Air, page 47) that of the radiation reaching the outer limits of the earth's atmosphere some 4o per cent of the longer ultraviolet rays and 20 per cent of the shorter ultraviolet rays reach high level moun tain resorts, and 23 per cent of the longer, and 7 per cent of the shorter ultraviolet rays reach sea level.
intensity of solar radiation of different wavelengths can be measured by means of a spectrobolometer, in which a beam of sunlight is first spread out into a spectrum, and the light within a narrow band of wavelengths is allowed to pass on to a platinum strip. The strip of platinum forms one branch of a Wheatstone's bridge. The opposite branch is formed by an exactly similar platinum strip which is shielded from radiation. The energy absorbed by the first platinum strip raises the temperature and consequently increases the resistance of the strip. The increase in resistance is accurately measured by a galvanometer in the bridge circuit and the amount of radiant energy can be deduced.
Ultraviolet rays have marked biological effects, in particular those of pigmentation of the skin and of destroying certain bacteria. References to recent researches will be found in the Annual Reports of the British Medical Research Council, pub lished by H.M. Stationery Office, London. Among recent results may be mentioned the discovery that ultraviolet rays between 0•28 and 0.30 microns are capable of activating ergosterol and producing vitamin D. (See a series of articles by various authors in a Supplement to Nature, April 2I, 1928; see also RADI OLOGY ; RADIO-THERAPY; SUNLIGHT-TREATMENT.)