DE LA RUE, WARREN (1815-1889), British astronomer and chemist, son of Thomas De la Rue, the founder of the large firm of stationers of that name in London, was born in Guernsey on the 18th of January 1815. Having completed his education in Paris, he entered his father's business, but devoted his leisure hours to chemical and electrical researches, and between 1836 and 1848 published several papers on these subjects. Attracted to astronomy by the influence of James Nasmyth, he constructed in 1850 a 13-in. reflecting telescope, mounted first at Canonbury, later at Cranford, Middlesex, and with its aid executed many drawings of the celestial bodies of singular beauty and fidelity. His chief title to fame, however, is his pioneering work in the application of the art of photography to astronomical research. In 1851 his attention was drawn to a daguerreotype of the moon by G. P. Bond, shown at the great exhibition of that year. Excited to emulation and employing the more rapid wet-collodion process, he succeeded before long in obtaining exquisitely defined lunar pictures, which remained unsurpassed until the appearance of the Rutherfurd photographs in 1865. In 1854 he turned his attention to solar physics, and for the purpose of obtaining a daily photographic representation of the state of the solar surface he devised the photo-heliograph, described in his report to the British Association, "On Celestial Photography in England" (1859), and in his Bakerian Lecture (_Phil. Trans._ vol. clii. pp. 333-416). Regular work with this instrument, inaugurated at Kew by De la Rue in 1858, was carried on there for fourteen years; and was continued at the Royal Observatory, Greenwich, from 1873 to 1882. The results obtained in the years 1862-1866 were discussed in two memoirs, entitled "Researches on Solar Physics," published by De la Rue, in conjunction with Professor Balfour Stewart and Mr B. Loewy, in the _Phil. Trans._ (vol. clix. pp. 1-110, and vol. clx. pp. 389-496). In 1860 De la Rue took the photo-heliograph to Spain for the purpose of photographing the total solar eclipse which occurred on the 18th of July of that year. This expedition formed the subject of the Bakerian Lecture already referred to. The photographs obtained on that occasion proved beyond doubt the solar character of the prominences or red flames, seen around the limb of the moon during a solar eclipse. In 1873 De la Rue gave up active work in astronomy, and presented most of his astronomical instruments to the university observatory, Oxford. Subsequently, in the year 1887, he provided the same observatory with a 13-in. refractor to enable it to take part in the International Photographic Survey of the Heavens. With Dr Hugo Müller as his collaborator he published several papers of a chemical character between the years 1856 and 1862, and investigated, 1868-1883, the discharge of electricity through gases by means of a battery of 14,600 chloride of silver cells. He was twice president of the Chemical Society, and also of the Royal Astronomical Society (1864-1866). In 1862 he received the gold medal of the latter society, and in 1864 a Royal medal from the Royal Society, for his observations on the total eclipse of the sun in 1860, and for his improvements in astronomical photography. He died in London on the 19th of April 1889. Entry: DE
Experimental photometry has been greatly developed since the pioneer work of Bouguer and Lambert and the subsequent introduction of the photometers of Ritchie, Rumford, Bunsen and Wheatstone, followed by Swan's in 1859, and O. R. Lummer and E. Brodhun's instrument (essentially the same as Swan's) in 1889. This expansion may largely be attributed to the increase in the number of artificial illuminants--especially the many types of filament- and arc-electric lights, and the incandescent gas light. Colour photometry has also been notably developed, especially since the enunciation of the "Purkinje phenomenon" in 1825. Sir William Abney has contributed much to this subject, and A. M. Meyer has designed a photometer in which advantage is taken of the phenomenon of contrast colours. "Flicker photometry" may be dated from O. N. Rood's investigations in 1893, and the same principle has been applied by Haycraft and Whitman. These questions--colour and flicker photometry--have important affinities to colour perception and the persistence of vision (see VISION). The spectrophotometer, devised by De Witt Bristol Brace in 1899, which permits the comparison of similarly coloured portions of the spectra from two different sources, has done much valuable work in the determination of absorptive powers and extinction coefficients. Much attention has also been given to the preparation of a standard of intensity, and many different sources have been introduced (see PHOTOMETRY). Stellar photometry, which was first investigated instrumentally with success by Sir John Herschel, was greatly improved by the introduction of Zöllner's photometer, E. C. Pickering's meridian photometer and C. Pritchard's wedge photometer. Other methods of research in this field are by photography--photographic photometry--and radiometric method (see PHOTOMETRY, CELESTIAL). Entry: 13
A curious circumstance, which may be explained by a duplex character of the matter forming a cometary tail, is the great difference between the visual and photographic aspect of these bodies. The soft, delicate, feathery-like form which the comet with its tail presents to the eye is wanting in a photograph, which shows principally a round head with an irregularly formed tail much like the knotted stalk of a plant. It follows that the light emitted by the central axis of the tail greatly exceeds in actinic power the diffuse light around it. A careful comparison of the form and intensity of the photographic and visual tails may throw much light on the question of the constitution of these bodies, but no good opportunity of making the comparison has been afforded since the art of celestial photography has been brought to its present state of perfection. Entry: A
Cinematograph films produced by ordinary photographic processes, being in black and white only, fail to reproduce the colouring of the subjects they represent. To some extent this defect has been remedied by painting them by hand, but this method is too expensive for general adoption, and moreover does not yield very satisfactory results. Attempts to adapt three-colour photography, by using simultaneously three films, each with a source of light of appropriate colour, and combining the three images on the screen, have to overcome great difficulties in regard to maintenance of register, because very minute errors of adjustment between the pictures on the films are magnified to an intolerable extent by projection. In a process devised by G.A. Smith, the results of which were exhibited at the Society of Arts, London, in December 1908, the number of colour records was reduced to two. The films were specially treated to increase their sensitiveness to red. The photographs were taken through two colour filters alternately interposed in front of the film; both admitted white and yellow, but one, of red, was in addition specially concerned with the orange and red of the subject, and the other, of blue-green, with the green, blue-green, blue and violet. The camera was arranged to take not less than 16 pictures a second through each filter, or 32 a second in all. The positive transparency made from the negative thus obtained was used in a lantern so arranged that beams of red (composed of crimson and yellow) and of green (composed of yellow and blue) issued from the lens alternately, the mechanism presenting the pictures made with the red filter to the red beam, and those made with the green filter to the green beam. A supplementary shutter was provided to introduce violet and blue, to compensate for the deficiency in those colours caused by the necessity of cutting them out in the camera owing to the over-sensitiveness of the film to them, and the result was that the successive pictures, blending on the screen by persistence of vision, gave a reproduction of the scene photographed in colours which were sensibly the same as those of the original. Entry: CINEMATOGRAPH