ULLMANINE A white water-colour pigment much favoured by retouchers of photographs for reproduction. It works well with the aerograph, photographs white, and does not change colour.
RAYS (Fr., Rayons ultra violets ; Ger., Ultraviolette Strahlen) A region of the spectrum lying beyond the visible violet, the commencement of which is sometimes known as the lavender rays. The research in this region of the spectrum has been much facilitated by the fact that the haloids of silver are extremely sensitive to it, and one has but to use a spectrograph fitted with quartz lenses and Iceland spar prisms to obtain a spectrogram of the same. Most of the work on the ultra-violet, however, is done with concave reflection gratings, which, forming a real image without the aid of lenses, introduce no absorp tive medium. The farthest yet reached by photographic means is A 1,181 by Victor Schu mann using a quartz spectrograph with silver iodide deposited on glass with but very little gelatine as binding agent and with the plate and spectrograph exhausted of air, as he found that both gelatine and air were rather strong absorb ents of the extreme ultra-violet. Ordinary glass
does not transmit beyond about x 3,400.
In the ultra-violet lies the region of the greatest photochemical action, and as has been pointed out in the description of the diffraction grating there is always an overlapping of the ultra-violet of the one order on the red of the preceding one, and therefore in spectrographic work this must be looked for and absorbed by the use of yellow filters. In ordinary photography, if anything like correct colour rendering is required also, the ultra-violet must be absorbed, for it is obvious that, as we do not see this region, any action on the plate would give a false rendering of the colour. Eder has given the following interesting table which shows the relative action of the ultra-violet rays in When using artificial lights such as the electric arc, the mercury vapour lamp, and magnesium ribbon, it may be considered that the bulk of the photochemical action lies in the ultra - violet. The practical absorbents of the ultra-violet are and filter yellow K.