Sept., I92[.] ('UleRENT Topics . 40 7

Preparation of Plates for Ultra-violet Light.--Pho¢ography offers the only satisfactory method for recording the phenomena of ultra-violet and infra-red light, inasmuch as these rays are invisible to the unaided eye. it is fortunate, indeed, that the ordinary silver salts are sensitive over so wide a range. It is true that many of the invisible rays can be observed by means of a fluorescent screen, but this does not furnish a permanent record. Research has shown a wide range of both forms of rays, and one serious interference with investigations is that ordinary glass is practically opaque to all vibra- tions above the violet, so that resort must be made to quartz, fluospar and other substances, either rare or dimcult to obtain in clear condition. The recent inventions, however, by which quartz can be cast in the form of clear plates and tubes will be of advantage in these researches. In a recent issue of the .loz~rnal de Physique ( 1:921, ii, I56 ) Duclaux and Jeantet describe a method of treating ordinary plates so as to increase greatly the sensitiveness to the higher vibra- tions. They had need of plates sensitive beyond 19oo angstr6m, and tried the procedure advocated by Schumann, but found it tedious and uncertain. Schumann plates are distinguished by the small pro- portion of gelatin, and it was thought that this condition could be secured by degelatin'izing to a great extent ordinary plates. Trials of various methods, such as immersion in warm water, acid solutions. digestive enzyms, were without success, but a simple and satisfactory procedure was devised.

The plate is placed horizontally in a dish with dihlte sulphuric acid (one volume of the strong acid to ten volumes of water) , and kept for four hours at to.ore temperature (about 77 ~ F. ) , the ten> perature being a little higher than this at the beginning and a little lower at the end. They are then removed to a dish in which they are washed by a very slow current of water, as the remaining gelatin is tender. Thir ty minutes will be a sufficient washing. They are then dried, which requires but little time on account of the small amount of gelatin present. Plates thus treated retain a thin layer of enmlsion poor in gelatin and uniformly spread on the glass. This deposit is extremely sensitive to ultra-violet rays, but is also very fragile, and the authors recommend that before developing the surface should be coated with a thin film of collodion, the plate being immersed in the developing bath before collodion is quite dry. :\lthongh most commercial plates are adapted fairly well for this procedure, it is likely that trial with many forms will show some more suitable than others. For rays of much greater wave-length than above noted, these plates are ten times more sensitive than the best plates prepared according to Schumann's method, and at least ~oo times as sensitive as the plate in its commercial form.

Another method for obtaining plates of high sensitiveness to short wave-lengths is by covering the emulsion with a layer of fluorescent substance. Such a substance absorbs, so to speak, the short waves and emits in turn waves of greater length, to which the gelatin is

408 CURRENT TOPICS. [J. F. I.

transparent, and thus permits an action on the silver compound, hence the impression is made as if the gelatin was not present. For this method, substances giving blue or violet fluorescence should be chosen, and they should be dissolved in a liquid that will not swell the gelatin, and is not absorbed by it, since the efficiency of the process depends on the fact that the fluorescent rays act before the light enters the gelatin film. Water is, therefore, net applicable. The authors obtained good results with a solution of eseulin in glycerol, but found most satisfactory results with lubricating oil. Many of the commercial forms of these have a distinct fluorescence due to hydrocarbons. It is sufficient to smear a few drops of such an oil over the emulsion by means of a wad of cotton. After exposure this film should be removed by means of ether or alcohol. A very thin fluorescent layer may be obtained by immersing the plate for a few minutes in a solution of the fluorescent oil in light petroleum or alco- hol and allowing the solvent to evaporate. These procedures are simple and effective. They enable the operator to secure photo- graphs of rays ranging from the extreme red to the limit of the ultra- violet. One slight defect is noted, a very small enlargement of the rays by irradiation, but this does not go beyond the twentieth of a millimetre.

The processes have been tried with many commercial plates, and the sensibility is found to be greater than with the sulphuric acid method. It is possible, indeed, to carry out an instantaneous spec - trography. Detailed results with certain metallic spectra are given in the paper. H . L .