March. I'Jli 



KX(3\VL1:DGE. 



117 



with a film dyctl with ;i solution of Nitrosodiinethylaniline. 



N O 

 Co Hjv^ which allows oiilv ultra-violet ravs to pass 



N (CH,), 

 through. To ilhiniiiiate the sitter, an arc light was employed 

 and the exposure with a rapid plate was five minutes. The 

 sharpness of the imase is not all that might be desired, but 

 this is no doubt due to the difficulty of ascertaining the correct 

 focus, together with the want of correction of the lens 

 (a portrait onel for these rays. Considering that the lens and 

 screen employed were composed of glass, the results obtained 

 are remarkable, as this material is very opaque to ultra-violet 

 rays : so that, as suggested by Professor Wood, quaitz. which is 

 e.\ceedingly transparent to these invisible rays, should be 

 employed instead, and lenses made of this material on the 

 surface of which a thin film of metallic silver has been chemi- 

 cally deposited to such an e.\tent that there is complete opacity- 

 to visible light and only those rays between three thousand 

 and three thousand two hundred transmitted. A great 

 peculiarity about photographs taken iti ultraviolet light is the 

 absence of strong shadows, and the rendering of some white 

 flowers almost black. .Again, their action in causing fluorescence 

 <ind phosphorescence are well known, and in all probability in 

 extremely short wavelengths they constituted X-rays. Two 

 French savants are also credited with the discovery that all 

 explosives are very unstable in ultra-violet light. Ultra-violet 

 light is also made use of in the sterilisation of water, a mercury 

 vapour lamp enclosed in a quartz chamber being employed 

 for the purpose. Further examples might be taken showing 

 the action of these rays, until we might almost feel inclined to 

 use the phrase '" L'ltra-xiolet light magic !" in all seriousness. 



EXPOSURE T.ABLE FOR M.\RCH.— The calculations 

 are made with the actinograph for plates of speed 200 H and 

 D, the subject a near one, and lens aperture F16. 



PHYSICS. 



By .Alfred C. G. Egi;rton, B.Sc. 



A NOTE ON FLUORESCENCE.— A large number of 

 substances when they absorb light give out light of different 

 wavelength. Thus, if a piece of uranium nitrate be held in a 

 beam of ultra-violet invisible light, it will become visible and 

 give out wavelengths ranging through the green portion of the 

 spectrum. If a little fluorescein is shaken on the surface of a 

 jar of water, the fluorescein as it dissolves gives beautiful 

 streaks of green in the liquid, though when the solution is 

 mi.xed up it transmits an orange light. Light passing through 

 the liquid has certain wavelengths absorbed from it, and the 

 energy thus .absorbed .goes partly to set the electrons within 

 the fluorescein molecule vibrating and give out light of their 

 own particular period — a vivid green light. Kaempf has 

 recently passed light through a solution of fluorescein, both 

 when it is rendered fluorescent by light from another source 

 and when it is not, and the intensities of the transmitted light 

 are then compared. No change in the intensity of the 

 transmitted light is caused by the fluorescence set up by the 

 other light source. Every absofbable wave has its energy 

 subtracted from the incident beam of light and the absorption 

 is independent of the intensity of the incident light. 



Many substances fluoresce, examples giving very beautiful 

 effects are eosin, magdala red. rhodamine, aesculine, quinine. 



It is probable that tlie list of fluorescent substances is very 

 large; only the: emitti'd light is in the infra-red and invisible 

 region of the spectrum. 



POSITIVE IONS FROM METALS.— When metals are 

 heated in vacuo, positive ions are given off. It is not yet 

 certain whether the ions are due to adsorbed gases or to traces 

 of impurities or to chemical reactions with the metal, residual 

 gases and glass, and the like, to which the metal is attached. The 

 adsorbed gases, according to recent work of Klemeusiewiecz, 

 appear to cause most of the effect ; but Professor Richardson 

 has shown that the marked positive ionisation effect obtained 

 with aluminium pliosphate is chiefly due to the presence of 

 sodium salts. 



GRAVITATIONAL FORCE.— The accurate measurement 

 of the force of gravity, which varies at different parts of the 

 surface of the earth, is a matter of some difficulty. The 

 counting of the number of swings that a pendulum of known 

 length will make in a known time is the principle on w-hich all 

 accurate methods rest. The pendulum is hung from a rigid 

 support in an air tight case from which the air is partially 

 exhausted, and with the aid of chronometer and electric flash 

 apparatus the time of half-swing of the pendulum is observed. 

 The support of the pendulum must either be quite rigid, 

 which is practically impossible to attain, or its movement 

 must be known. The support should be mounted on a con- 

 crete pier extending six-feet into the ground, and its motion 

 determined by means of an optical instrument detecting very 

 slight movement, called an interferometer. The measurement 

 of gravity at sea can only be accomplished by a comparison 

 of the height of the mercury barometer and of the pressure 

 calculated from the temperature at which water boils. Where 

 the force of gravity is greater, the height of the mercury 

 column will be slightly less than the height calculated from the 

 boiling-point. 



EMISSION OF ELECTRICITY FROM HEATED 

 CARBON. — Drs. Harker and Kaye have found that if two 

 insulated carbon electrodes are placed in a high temperature 

 furnace, and one of the carbon poles is cooled, that a current 

 is set up between the two through an ammeter connected fo 

 them. By cooling one of the electrodes a permanent current 

 is maintained of 0-8 ampere nearly. The currents are pre- 

 sumably due to discharge of negative electrons from the hotter 

 electrode, a current being thus able, through the ionisation of 

 the gas in the furnace, to pass from one electrode to the other. 

 This effect appears to lend support to the ideas held about the 

 action of an oscillatory Dudell arc. 



PHOTOELECTRIC FATIGUE.— E.xperiments made by 

 H. S. Allen and by J. Robinson on the fatigue shown by 

 various metals in the production of negative ions by the 

 stimulus of ultra-violet light, are somewhat at variance. 

 Robinson's results show that such fatigue exists. Ultra-violet 

 light passes through a window of quartz, and falls on a polished 

 plate of zinc. It then charges itself with positive electricity if 

 insulated. The potential can be measured by an electrometer. 

 The rate of charging is diminished when the plate is exposed to 

 ultra-violet light at zero potential at the commencement ; but 

 if the plate is '" fresh," i.e.. kept in the dark for some time 

 previous to exposure, the rate of charge is much greater. 

 Hallwach's theory of the cause of photoelectric fatigue is that 

 when electrons leave the surface of a metal, the stream will 

 draw up out of the metal occluded gas molecules, which will 

 accumulate at the surface and prevent the electrons getting 

 through, since these slow-moving electrons are easily absorbed 

 by gases. If the plate is charged positively no electrons leave 

 the surface, and there will be no accumulation of gas there, 

 and therefore no fatigue. Recovery from fatigue is supposed 

 to be due to the gas becoming more unilormly distributed 

 within the metal. Experiments were also carried out with 

 aluminium. 



AFTERGLOW OF ELECTRIC DISCHARGE.— 

 Professor Strutt has recently given an account to the 

 Physical Society of the continuation of his work on the 



