PHYSICS, PROGRESS OF, IN 1890. 



715 



only from approximately normal incidence. His 

 results differed somewhat from those of Kundt. 



A. Gleichen (" Zeitschriit fur Physik") finds 

 that when a pencil of rays falls on a prism 

 whose refracting edge is perpendicular to the 

 axis of the pencil, the astigmatic difference is 

 independent of the distance of objects only in 

 the case of minimum deviation. 



Dispersion. A. Winkelman (Wiedemann's 

 " Annalen," July) has investigated the anoma- 

 lous dispersion of colored glass. Didymium 

 glass shows two regions of such dispersion one 

 in the red and one in the green. Uranium glass 

 has three in the red, between the yellow and 

 green, and in the edge of the blue. Cobalt glass 

 has two in the edge of the red toward the blue, 

 and in the green. Ph. Barbier and L. Roux 

 (Paris Academy of Sciences, May 27) find that 

 the dispersive power of the alcohols of the fatty 

 series is a continued function of their molecular 

 weight ; and, contrary to what is true of the 

 aromatic series, the dispersive power increases 

 with increase of the molecular weight. The ab- 

 sence of hydrogen also increases the dispersion. 



Photometry. A. Cerova (Carl's " Repertori- 

 um," page 109) by means of a spectrophotometer. 

 has compared the intensity of sky light at five 

 different wave lengths with that of a carcel lamp. 

 He shows that, sky light has an excess of blue. 

 If the intensity for wave length -0 S 565 be called 

 100, then the results are as follows : 



The excess of blue diminished from morning 

 till noon and then increased. It differed from 

 day to dav. Edmund J. Spitta (London Royal 

 Society, Dec. 5, 1889) has devised an improved 

 wedge photometer. When a point of light is 

 compared with a disk by the ordinary wedge 

 photometer, a cause of uncertainty arises, owing 

 to the fact that the light from different parts of 

 the disk passes through different thicknesses of 

 glass. Spitta uses two glass wedges, which slide 

 past each other, and thus give a field of uniform 

 intensity where the points overlap. Lion (" La 

 Nature," Sept. 6) has devised a nitrogen-iodide 

 photometer. He finds that equal surfaces of the 

 iodide, preserved under its mother liquor and 

 exposed for equal times to lights of equal in- 

 tensity, evolve equal volumes of nitrogen. In 

 the instrument the iodide is contained in two 

 vessels connected by a differential manometer. 

 Seguy and Verschaffel (Paris Academy of Sci- 

 ences", Sept. 1) have devised a photometer on the 

 principle of a Crookes radiometer, but the disks, 

 instead of being allowed to revolve, are sus- 

 pended to form a torsion balance, and are pro- 

 vided with a needle, which, by its deflection, 

 measured in a divided arc, shows the intensity of 

 the lignt. 



Standard of Liqlit.Dv. Brodhun and Dr. 

 Lummer (Berlin Physical Society, March 21) 

 have .experimented on electric glow lamps fed 

 by accumulators, with a view to using them as 

 standards, and find that they vary by only 1 per 

 cent, in two hundred hours. 



Optical Instruments. The " periscope, " us-d 

 on submarine war vessels, is a simple application 

 of the principles of reflection. As u*ed on the 

 French torpedo boat " Gymnote," it is a vertical 

 telescopic arrangement with a lenticular total- 

 reflection prism at top. After reflection in this, 

 the rays converge to a point and are received by 

 a lens whose principal focus coincides with thi> 

 point; the vertical cylindrical beam thus formed 

 meets a mirror inclined at an angle of 45, 

 and is directed horizontally to an eye-piece. 

 Thus, when the total reflection prism is above 

 water and can be turned toward any part of the 

 horizon, any object on the water may be seen at 

 the eye-piece in the boat below. 



Electricity. Electric Wares. The discovery 

 of electric radiation by Hertz (''Annual Cyclope- 

 dia," 1889, page 694) has given a great impulse to 

 the study of electro-magnetic induction. Many 

 new waysof producing or investigating this radia- 

 tion have been devised. G. Bartaniek (Beibldtler 

 to Wiedemann's "Annalen " XI V, 654) uses an in- 

 candescent lamp whose carbon filament has been 

 broken, for showing the spark in the " resonator." 

 Dr. Ignatius Klemencic has investigated the vi- 

 brations by means of a thermo-element soldered 

 between the ends of the secondary inductor. H. 

 Classen, by blowing a stream of air between the 

 terminals of a Ruhmkorf coil, produces a sharp 

 crackling spark which can be used instead of that 

 of the " vibrator." Ernest Lechner (Vienna 

 "Berichtc," May 8) investigates electric reso- 

 nance by leading wires from insulated plates op- 

 posite the terminal plates of a Hertz's arrange- 

 ment. Over the ends of these wires, when they 

 are placed parallel, he holds a tube filled with 

 rarefied air, which becomes luminous. If the 

 parallel wires be connected by a wire link, the 

 luminosity, in general, ceases, but it reappears 

 periodically as the link is slipped along the 

 wires. The places where the link causes it to 

 reappear are evidently the "loops" of station- 

 ary vibrations in the wires. His experiments 

 confirm those of Hertz, except that he finds for 

 the velocity of the radiation a quantity within 

 2 per cent, of that determined by Maxwell, 

 with whom Hertz did not agree on this point. 

 Dr. Rubens and Dr. Ritter (Berlin Physical So- 

 ciety, March 7) have investigated electric radia- 

 tion by means of the bolometer. They con- 

 structed a Wheatstone's bridge, two of whose 

 arms were secondary bridges. If a current pas- 

 ses through one, the resistance is altered by rise 

 of temperature, and the galvanometer gives a 

 throw. The same effect is produced by a wave 

 of electric radiation ; hence, the amplitude of 

 such a wave can be measured. Their experiments 

 with a polarizing wire grating show that there is 

 a constant relation between the intensity of the 

 rays that pass it and the angle of inclination of 

 the wires. When the wires are at right angles 

 to the vibrator, 98 per cent, of the energy is re- 

 flected. Edouard Sarasin and Lucien tie la Rive 

 (Paris Academy of Sciences, Jan. 13), in discov- 

 ering what they term "multiple resonance, 

 throw doubt on 'Hertz's hypothesis; and Cornn 

 asserts that we must receive all of the German 

 physicist's theoretical inferences with caution. F. 

 T. Trouton, however, has observed the same thing, 

 and explains it conformably with the received 

 theory. He found that resonators of different 



