Expressed thus, the wave-lengths of visible radiations extend between 4,000 for the extreme violet and 7,000 for the extreme, easily visible red (with special precautions, trained observers have been able to see as far as Fig. represents the distribution of wave-lengths between the different colours in a normal spectrum, the wave-length scale being equally graduated throughout. In the figure are also marked the positions of the black bands of the solar spectrum, called the Fraunhofer lines, denoted by the letters A to H, and constituting a series of reference marks which are sometimes used to denote the different spectral regions in cases where extreme precision is unnecessary. The frequency (number of waves per second) is constant for each radiation, irrespective of the medium, and is approximately expressed for yellow light by the figure 5, followed by 14 zeros. These numbers are not, as might be thought, simply theoretical speculations, but the expression of practical measurements ; the wave-length is frequently used as a measure of length in precision work and even in certain industrial measurements.
3. Luminosity of Different Spectral Colours. For an observer who is not colour-blind (people 1 Note that the interval between the extreme visible radiations is less than a musical octave.
presently, of special interest in regard to the choice of illuminants for the dark-room.
About 95 per cent of the visual effect of the spectrum is confined between the lines C and F, whilst photographic action on ordinary emul sions is limited only to those radiations of wavelength less than that of the F line. This peculiarity has important consequences in the photographic rendering of different colours.
4. The Ultra-violet and the Infra-red. The radiant energy of the sun and various artificial illuminants is not limited to the visible region of the spectrum, but covers a range, which is actually known, of at least 15 octaves.
If, in the region beyond the speetrum violet, pieces of paper impregnated with fluorescein or rhodamine (fluorescent substances) are placed, it will be seen that they emit respectively green and red light. This effect they also show in the blue-violet but not to such an easily observable extent. The same phenomenon can be shown with crystals of uranium nitrate, with screens covered with barium platinocyanide (used in radioscopy), calcium tungstate (X-ray intensi fying screens), or certain preparations of zinc sulphide. In order to demonstrate the existence of these invisible radiations a spectrum need not necessarily be used. By projecting on to a screen the image of an electric arc and then placing in the path of the beam a piece of special black glass (Wood's glass) which absorbs all the visible light whilst transmitting the ultra violet, the image of the arc can be made to re-appear by placing at the point where the visible image existed one of the fluorescent screens mentioned above.
The glasses generally employed in the con struction of optical instruments transmit the ultra-violet down to about 3,50o A.U. The limit
extends further to about 3,20o A.U. in the case of certain special glasses (livid). Thanks to the absorption of our atmosphere, the solar spectrum ends at about 3,000 A.U., which fact protects us from the very dangerous physiological effects of the shortest wave-length radiations such as are produced by arcs between metal electrodes and transmitted by quartz (rock crystal) down to about 2,000 A.U., which is also approximately the limit of transparency of gelatine and air. By means of suitable apparatus (reflection gratings in vacuum), and by using sensitive surfaces without gelatine, it has been possible to study photographically the ultra-violet down to about ioo A.U., where it joins the X-rays.
Quartz lenses, used in conjunction with filters which transmit only ultra-violet, are employed in certain special applications of photography. A filter suitable for this purpose is a thin film of silver.
In photographs obtained with these radiations alone, glass objects appear completely opaque ; certain white flowers and pigments are indis tinguishable from pure blacks, and, further, no background and no shadows appear in a land scape photographed in bright sunshine. The photograph looks as if it were taken in a dense fog (R. W. Wood, 191o).
The infra-red, which extends the visible spec trimi beyond red, was of no photographic interest for a long time, its effects being chiefly thermal. The only known means for infra-red photography was an indirect method based on the fact that these radiations discharge almost instantaneously phosphorescent bodies. By uni formly exciting a phosphorescent screen by ultra-violet, forming on it an infra-red image and then applying it against a sensitive layer, a positive image (§i8) is impressed by the residual phosphorescence.
Since the discovery of sensitizers (223) for infra-red which are sufficiently easy to handle (E. Q. Adams and H. L. Haller, '919; H. T. Clarke, 1925), it has been possible to place on the market plates and films for infra-red photo graphy up to io,000 or 13,000 A.U. These sensitive emulsions must be kept and manipu lated with special care. The extreme trans parency of the atmosphere for infra-red has enabled photographs to be taken, with these rays only, of landscapes up to 331 miles distance from a high viewpoint. If all radiations of wave length below q,000 A.U. are eliminated, green foliage is reproduced as white as snow, owing to the intense fluorescence of chlorophyll with a maximum near 7,40o A.U. (C. Dare and A. Rally, 1935), and as the blue of the sky is ren dered as black and there is no shadow detail the landscapephotographs thus taken in sunshine with a clear sky give an impression similar to that of a photograph taken by moonlight. 5. Natural and Pigmentary Colours. By the colour of an object is always meant the colour it appears when seen in white light, but this colour depends essentially on the nature of the light which illuminates it.