PHYSICS, IMIOCIIIKSS OF, IN 1893. 



621 



any I'iirticulnr grating can ho made, -o tlial they 

 need not prevent accurate oler\ation. Crovu 

 (I'uris Aciulriiiy of Science. March 2?) has in- 

 vr.-i i-ated tin- spectra produced by a grating of 

 biehromated gelatin <>n silvered glass. Tin- in- 

 terfcivnee ui' tin- sped ruin from the gelatin 

 with that from its ivllfction in the glass pro- 

 duced straight or curved hands, sometimes of 

 great beauty. No such hands were observed wit h 

 trails-parent gla-s. since its refractive! index is 

 the sjime as that of the gelatin. W. B. Croft 

 (London Physical Society, Dec. 9) has invoti- 

 galed the orders of colors in the so-called New- 

 ton's scale. The standard treatises imply that 

 the number of hands in each order is the same 

 as that of the order, but Mr. Croft finds that the 

 lir.-t three orders have one band each, and the 

 fourth three bands. The explanation is based on 

 the numerical position of the wave lengths in 

 the visible spectrum that is, on the answer to 

 the question whether a multiple of one wave 

 length can be a multiple of another. B. W. 

 Snow (Wiedemann's " Annalen," No. 10) has in- 

 vestigated infra-red spectra, using a modified 

 bolometer and a galvanometer with a delicate 

 quartz fiber. In the solar spectrum he finds 

 numerous bands composed of fine lines, as far 

 as wave length 20,620, the principal ones being 

 between 7,000 and 7,700, 7,850 and 8,600, 9,000 

 and 10,000, 10,750 and 11,600. 13,700 and 15,000. 

 The infra-red spectra of 5 metals sodium, po- 

 tassium, lithium, rubidium, and caesium were 

 examined, and maxima found as follow : For 

 sodium, at 8,180, 11,270, 12,400, and 18,360 ; for 

 potassium, at 7,670, 10,820. 11.580, 12,250, and 

 14,610; for lithium, at 8,070; for rubidium, at 

 7,910, 9,980, 13,120, and 14,760; and for cwsium, 

 at 8,380, 8,820, 9,980, 13.270, and 14,530. 



Interference Refractometer. This instrument, 

 so named from its first uses, and called also by 

 Michelson in one form a wave compiler ("An- 

 nual Cyclopaedia," 1892, p. 639), promises to be- 

 come one of the most valuable physical instru- 

 ments. In a description (' Nature," Nov. 16) 

 Prof. Michelson asserts that for certain pur- 

 poses of measurement it surpasses and will super- 

 sede the telescope, the microscope, and the 

 spectroscope. This is due to the fact that the 

 limit of accuracy in setting a cross hair of a 

 microscope on a fine-ruled line is about '05 mi- 

 cron, whereas the middle of an interference 

 fringe, if it be broad and clear, can be determined 

 to within gV of the width of the fringe, or -01 

 micron. In actual practice it has been found 

 that the average error is only about '01 of a 

 fringe, or -003 micron. Thus, what is lost in 

 distinctness is more than made up in accuracy 

 in measurements of position. Similar considera- 

 tions hold in the measurement of angles by the 

 telescope. As a spectrometer, for which the 

 form called a wave compiler is used, the instru- 

 ment possesses no advantages over the ordinary 

 spectroscope in clearness of definition ; its great 

 advantage lies in the ease with which the dis- 

 tribution of light can be inferred in a spectrum 

 beyond the powers of either instrument to re- 

 solve perfectly. Not the least of the uses of the 

 instrument is the comparison of material stand- 

 ards of length with that of a light wave, thus 

 furnishing a permanent metrical standard, the 

 same for all parts of the known universe. 



Electricity. The Voltaic Cell. Klrter and 



(ieitel, of Wolfetibiitlel r Xeit >chrift fflr physi- 

 kalische und chcniischc L'nterricht," quoted in 

 " Nature," .Ian. 5). have devised an apparatus for 

 measuring the difference of potential of a cell, 

 based on the principle of Thomson's water-drop 

 influence machine. Two jets enter 2 insulated 

 metal vessels through 2 rings, one of them con- 

 nected with the positive pole of the cell. The 

 jet is charged negatively, and communicates its 

 charge to the vessel and by induction to the 

 other ring which induces on the other jet a pos- 

 itive charge that accumulates on the second 

 vessel. 



rnitirizationofCell, Bouty (Societe Francaise 

 de Physique) has experimented on polarization 

 in melted electrolytes, dilute solutions of salts, 

 and solid electrolytes. When a point electrode 

 remains in a melted electrolyte for twenty-four 

 hours it has for a given time a constant initial 

 capacity of polarization which increases rapidly 

 with the time, while the maximum polarization 

 decreases. John Daniell (" Nature, Sept. 28) 

 finds that when thin partitions of metal are im- 

 mersed in a cell polarization takes place when 

 the thickness of the partition exceeds a certain 

 definite amount. With a current strength of 

 not more than O'l ampere per square centimetre 

 this thickness is between 0-00009 and 0-0004 

 millimetre for gold, and between 0*00015 and 

 0-0002 for platinum in sulphuric acid, copper 

 sulphate, and common salt. 



Resistance. Fernando Sanford, of Leland 

 Stanford University (" Philosophical Magazine," 

 January), shows that the resistance of a metallic 

 conductor varies with the dielectric in the field 

 of force, but that probably only that part in di- 

 rect; contact with the wire is concerned in the 

 action. The resistance of a copper wire in air 

 being taken as unity, the resistance of the same 

 in various dielectrics was as follows: Petroleum, 

 1-0018; bisulphide of carbon, 10009; benzene, 

 0-9998 ; absolute alcohol, 0-9981 ; wood alcohol 

 and petroleum, 0-9973. J. H. Gray and J. B. 

 Henderson (London Royal Society, March 2) 

 have investigated the effect of stress on the 

 electrical resistances of copper, iron, and steel, 

 and conclude that for practical purposes they 

 are not affected thereby. The smallest impurity 

 produces a greater change in resistance than the 

 severest treatment, an impurity of 0-4 per cent., 

 for instance, lowering the conductivity by 13-5 

 per cent. As the result of experiments made at 

 the Phvsikalisch-Technische Reichanstalt, Ber- 

 lin (" Nature," Jan. 5), a mercury standard re- 

 sistance has been constructed that requires no 

 renewal. V-tubes are filled with mercury in 

 vacua, sealed by fusion, and into each end are 

 fused three platinum wires, connecting respec- 

 tively with the main circuit, the secondary cir- 

 cuit, and the galvanometer. The wire is so' thin 

 that it conducts practically no heat. Each 

 standard is immersed in petroleum in a brass 

 box, and the box is placed in ice and water. 

 Van Aubel ("Journal de Physique." September), 

 in experiments on the resistance of bismuth, finds 

 that neither sudden cooling nor compression 

 a fleets it much. When the metal is pure, the 

 resistance increases with rise of temperature, 

 and this increase is regular between and 100, 

 but a trace of impurity alters the property. 



