PHYSICS, PROGRESS OF, IN 1901. 



533 



selective absorption a rise of temperature moves 

 the absorption curve toward the longer wave- 

 lengths, and sometimes also increases the width 

 of the absorption band without sensibly altering 

 the magnitude of the maximum absorption. In 

 metals the absorption is not influenced by tem- 

 perature within the limits 10-3GO. Platinum 

 shows no influence up to 800. The author shows 

 that these experiments explain the influence of 

 temperature on the refractive index and the dis- 

 persion on the assumption that the dielectric con- 

 stant always diminishes with increase of tempera- 

 ture. 



Refraction. F. Pockels (Physikalische Zeit- 

 schrift, Aug. 31) has investigated this problem 

 experimentally and finds that in polarized light 

 the component polarized perpendicular to the di- 

 rection of the pressure is retarded relatively to 

 that parallel to the pressure, and, contrary to the 

 usual result, the glass becomes positively double 

 refracting. 



Interference. C. Barus (Science, 12, 1900) notes 

 that the interferences observed on viewing one 

 grating or piece of gauze through another are 

 very complex. If two gratings are placed at a 

 distance apart along an axis, and the first illu- 

 minated by a strong diffuse light, the second will 

 project a real image of the former grating at 

 definite points on the axis. When these images 

 are looked at by the eye in the proper position, 

 they appear as magnifications of the first grat- 

 ing, often enormously large, the size increasing 

 with the distance of the focal plane from the 

 projecting grating. The indefiniteness of focus 

 when viewed by the normal eye is due to its 

 power of accommodation, and the size is an illu- 

 sion, for the eye is adjusted for an infinite dis- 

 tance and locates the image of unknown position 

 there. 



Magneto-optics. H. A. Lorentz (International 

 Physical Congress at Paris, Report, 3, 1900), in 

 a discussion of the various theories of the influ- 

 ence of a magnetic field on the emission of light, 

 shows how, if a radiating molecule is a material 

 system of a certain number of degrees of free- 

 dom, there results a spectral ray which splits up 

 under the action of a magnetic field into a num- 

 ber of components corresponding to the frequen- 

 cies that are equal apart from that magnetic field. 



Spcctroscopy . N. E. Dorsey (Astrophysical 

 Journal, September, 1900) has made observations 

 to determine whether in the refraction of light 

 the medium requires time to reach a steady state 

 that is, w'hether a short train of w r aves is re- 

 fracted in the same way as a long train of the 

 came period. If the initial disturbance 'is re- 

 fracted differently from a continuous one, then 

 the lines as seen in a prism instrument should 

 be broadened or rendered hazy on one or both 

 sides as compared with the same lines in the 

 grating spectroscope, but no such effect could be 

 observed either with a continuous source of light 

 or an electric spark. Haschek (Sitzungberichte 

 of the Vienna Academy, January-March), in an 

 investigation of a possible connection between 

 the wave-lengths of spectral lines and the mode 

 of production of the incandescent vapor used to 

 produce them, finds that displacements of lines 

 in the arc and spark spectra are common, and, 

 under certain circumstances, reach considerable 

 magnitudes, but that the wave-lengths depend in 

 a high degree on the momentary conditions of the 

 experiment, and these can not always be con- 

 trolled. The wave-lengths of the lines are to a 

 great extent dependent upon the consumption of 

 energy, and as this is greater with the trans- 

 former, some lines are strongly displaced toward 



the red. (See also Keiklmf-. < \; n ta under 



ELECTRICITY, GVww, bclov 



Telescopy. J. Renton i N :. . i , , ; , ,| j s . 



cusses a hitherto unexplahx i 

 paring the zenith distance <>) ;i 

 direct observation with that obi.i 

 tion in a pool of mercury, lie ;in 

 difference of temperature in the st;i-n,': 

 in the telescope tube. The upper M<|< < ; 

 is cooled by radiation, while the lower ; 

 tected, causing a transverse gradient of teni] 

 ture in the air inside. The rays passing through 

 the tube will thus be bent upward. The erroi 

 is important because it affects the zenith distances 

 of polar stars, and hence the deduced results for 

 latitude. The writer recommends that means 

 should be used for circulating the air in the tube. 



Phosphorescence, Radio-activity, etc. (See also 

 Rontycn Rays under ELECTRICITY.) The subject 

 is reviewed in the Revue Scientifique, Sept. 8-15, 

 1900, by Gustav le Bon. He asserts that most 

 bodies under the influence of light become radio- 

 active, emitting effluvia or Becquerel rays. He 

 admits no real distinction between fluorescence 

 and evanescent phosphorescence. The spectrum 

 from blue to ultra-violet excites phosphorescence; 

 from green down to infra-red it tends to extin- 

 guish it; in an intervening portion it excites up 

 to a certain intensity and reduces all above that 

 intensity. The temperature at which phosphores- 

 cence begins is always far below incandescence, 

 and phosphorescence is a " cold " light, but too 

 weak for practical use. This weakness is not the 

 cause of the apparently low temperature; we do 

 not know anything about the infra-red region of 

 the phosphorescence spectrum. Phosphorescence 

 too faint to be seen may continue for eighteen or 

 more months without any heating. Even when 

 invisible phosphorescence has died out, certain 

 dark radiations may reexcite phosphorescence up 

 to the point of visibility. There is, therefore, 

 some " residue " permanently left in the body, as 

 the result of the momentary exposure to light; 

 Taut this can be dissipated by heat. It is the 

 impact of the particular dark waves, not the heat 

 evolved, that produces the temporarily revivity- 

 ing effect and the partial or complete exhaustion 

 of the residue. The " heat-waves " are not neces- 

 sarily infra-red; they may be so when the source 

 is at a low temperature; but when the compres- 

 sion of the whole infra-red into a small space 

 in the ordinary spectrum is taken into account, 

 the maximum heating power in sunlight is found 

 to be near line D. The " residue " mentioned 

 above does not seem to be stored luminous ener- 

 gy, but may consist of decomposition products 

 that unite when the temperature favors combina- 

 tion. Minerals cut in the dark from the interior 

 of very large lumps, and which have therefore not 

 seen the light for ages, shine upon being heated 

 as w r ell as if they had recently been exposed to 

 light. They appear to contain the products of de- 

 composition referred to. The " residue " when 

 stimulated into activity, say by a temperature of 

 100, becomes exhausted if this temperature is 

 kept up, but then a temperature of 200 will 

 bring a fresh portion into play, and so on up 

 to say 500 or 600. Heating to 100 for months 

 will not bring out that portion of the residue 

 which comes into activity only above 100, and 

 so forth. A body incompletely exhausted at 500 

 and then cooled down and reheated to 200 will 

 not shine, for all that portion of the residue i 

 exhausted. Phosphorescence by shocks or blows 

 appears not to be due to localized hi<rh tempera- 

 ture, but to a conversion of energy similar to 

 an explosion. Phosphorescence under Rontgen 



