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PHYSICS, PROGRESS OF, IN 1892. 



Phosphorescence. M. Lenard has invented a 

 phosphoroseope for the observation of bodies 

 illuminated by the electric spark. The results in 

 most cases are similar to those obtained in 

 Becquerel's instrument, but arragonite, which 

 is there invisible, gives a faint reddish phosphor- 

 escence by electric spark. Asaron, which is 

 bright in a Crookes tube and glows in the ultra- 

 violet spectrum, is absolutely dark. (See also 

 Color, under " Light," below.) 



Fluorescence. G. Salet (Paris Academy of 

 Sciences, Aug. 1) has observed the diagonal 

 spectrum of Stokes's experiment by receiving 

 the spectrum from a quartz prism on a soret 

 eyepiece and projecting it thence transversally 

 on the slit of a second spectroscope. 



Photometry. Charles Henry (Paris Academy 

 of Sciences, Oct. 24) has invented a photopto- 

 metric photometer for very feeble light. It 

 uses phosphorescent sulphide of zinc. The law 

 of loss of brilliancy being determined, it is illumi- 

 nated for a given time by a standard magnesium 

 light, and then the time is observed which elapses 

 before its brilliancy has fallen to that of the light 

 to be measured. 



Refraction. Jaques Chappuis (Paris Academy 

 of Sciences, February) finds that the indices of 

 refraction of sulphurous acid and methyl chlo- 

 ride at C. and under maximum vapor pressure 

 are respectively 1-3518 and 1-3533 for the D line. 

 Rubens and Snow (Wiedemann's " Annalen," 

 No. 8) find by the bolometer that fluorspar has 

 a less dispersion than rock salt in the violet, but 

 enormously greater in the infra-red, and hence 

 is specially adapted for the production of pris- 

 matic heat spectra. D. Shea (ibid., No. 10) has 

 measured the refraction and dispersion of several 

 metals by Kundt's method (' Annual Cyclo- 

 pedia," 1889, p. (597). His platinum prism was 

 made by the disintegration of foil. His results 

 were as follow : 



Reflection H. E. J. G. Du Bois (Wiede- 

 mann's "Annalen," No. 8), in a paper on reflection 

 and transmission in reolotropic media, describes 

 experiments with gratings of bright silver wire, 

 platinum film, scratched metal, and scratched 

 glass, and with crystals of cobaltine and pyrites. 

 In the silver-wire gratings, light polarized in a 

 plane normal to the direction of the wires was 

 transmitted in greater proportion than that 

 polarized in the direction at right angles. The 

 contrary was true with the scratched glass and 

 metal. 



Interference. Dr. Zehnder (Berlin Physical 

 Society, Nov. 6) has devised a simple differential 

 refractor by which the two rays which are ulti- 

 mately to be made to interfere are kept 50 to 100 

 centimetres apart, so that they can be subjected 

 to varying experimental conditions. 



Magnesium as a Source oj Light. Frederick J. 

 Rogers (" American Journal of Science, April) 

 concludes (1) that the spectrum of incandescent 

 magnesium approaches nearer than any other 

 artificial light to that of sunlight, and (2) that its 



temperature is about 1,340 C., lying between that 

 of the Bunsen burner and the air-blast lamp, 

 though its spectrum corresponds to a tempera- 

 ture of 5,000 C. The radiant energy emitted is 

 4,630 calories per grain, or 75 per cent, of the 

 total heat, while that of gas is only 20 per cent. 



Color. Nichols and White (" Philosophical 

 Magazine," November, 1891) find that the color 

 of a pigment arises from light reflected (1) from 

 the surface and (2) from interior faces. The 

 surface light is nearly white. Heating dimin- 

 ishes the reflective power, especially in the 

 regions of greatest refrangibility with the result 

 of shifting the color toward the red. In chromic 

 oxide and zinc oxide, however, it is shifted toward 

 violet. Later (January, 1892), from further ex- 

 periments on zinc oxide they conclude that it is 

 highly luminous above 880, probably from 

 phosphorescence. A. Noble (" Nature," March 

 24) states that the first visible color of a thin iron 

 sleeve gradually heated by a bar within appeared 

 as described by different observers, to be " gray- 

 white," " white like phosphorus in the dark," or 

 " white with a dark shade." T. C. Porter (ibid. 

 April 14) reports a similar experiment on the 

 carbon filament of an Edison incandescent lamp. 

 There were 25 observers, to all of whom the first 

 visible light appeared very pale. To 13 it was 

 " yellow " ; to 7 " faint pink " ; to 2 " bluish 

 white." All agreed that it passed through orange 

 before reaching crimson. Prof. S. P. Thompson 

 (London Physical Society, Feb. 12) finds that any 

 non-monochromatic color can be split into two 

 tints, which he calls supplementary. Of these, 

 one may always be grayish, thus verifying 

 Abney's law that any color may be produced by 

 diluting a spectrum color with white. W. Ost- 

 wald (Royal Society of Saxony, xviii, 281) finds 

 that the absorption spectrum of a dilute solution 

 of a salt is the sum of the spectra of its ions. 

 In the cases where one ion is more absorbent, 

 the spectra of all salts formed from such ions 

 and the same colored ion are precisely the same. 



Iridescent Colors. Alexander Hodgkinson 

 (" Proceedings of the Manchester [Eng.] Literary 

 and Philosophical Society ") states that long ex- 

 perience in the examination of iridescent objects 

 has shown him that almost without exception 

 their colors are due to interference produced by 

 thin plates, and vary with the inclination of the 

 plate to the light. He proposes that, to secure 

 uniformity in the description of such objects, a 

 particular incidence be always chosen : 90 being 

 the best, because then the trouble of measuring 

 an angle is obviated, it being only necessary to 

 be sure that the reflected ray coincides with the 

 incident. This is easily done by employing a 

 perforated mirror, which Mr. Hodgkinson finds 

 appears to change iridescent bodies wonderfully 

 in appearance. For instance, the crest of a hum- 

 ming bird, which to the naked eye is resplendent 

 with all the colors of the r'ainbow, appears 

 through the mirror of an unvarying red tint, and 

 hence is described as ''iridescent red." Other 

 objects, similarly, are iridescent blue or green. 

 A piece of iridescent iron ore displays different 

 colors in its different parts, but these remain un- 

 changed when the specimen is moved and do not 

 shift as they do when viewed with the naked eye. 



Absorption. W. Peddie (British Association) 

 finds that the coefficient of absorption is not 



