C. GENERAL PHYSICS. 175 



some other substance, is called the specific inductive capaci- 

 ty, or, more briefly, the dielectric constant of the insulating 

 substance. The condition in which the isolating body is 

 found is called dielectric polarization, and the phenomena 

 of dielectric polarization are quite distinct from those of 

 electric conduction. Bolltzman has undertaken an extensive 

 series of experimental investigations to show that the phe- 

 nomenon known as dielectric action, at a distance, actually 

 exists, and he has determined its amount. To this end, he 

 fastened spheres made of different isolating substances to 

 one of the arms of a very delicate balance, and measured the 

 traction experienced by it for an electric metal sphere brought 

 in the neighborhood. The minute interval of time occupied 

 in order to produce the maximum effect showed that the trac- 

 tion results from a momentary dielectric polarization of the 

 molecules. Sitz.-Bcr. Boy. Acad. Sc, Vienna, LXVIIL, 81. 



OPTICAL PROPERTIES OF TIIIX METALLIC FILMS. 



In our previous volume are given the curious and val- 

 uable results obtained by Mascart on the phenomena of the 

 polarization of light reflected from extremely thin films of 

 silver and other metallic substances. The thickness of these 

 films varied between one hundredth part of the length of a 

 wave of light and the relatively much greater thickness of 

 about one one thousandth of an inch. The observations of 

 Mascart have been recently repeated by Quincke, who has 

 made a very thorough study of the question of the prime angle 

 of incidence for the different Fraunhofer lines. By this angle 

 is understood one such that polarized light, after reflection 

 from a plane surface, will be resolved into two components 

 which are polarized, respectively perpendicular and parallel 

 to the plane of incidence, and having a difference of phase of 

 a quarter -wave length. Quincke finds that variations of 

 temperature and of pressure in polishing frequently alter the 

 optical qualities of a mirror very considerably ; so that it is 

 probably much more difficult to obtain faultless reflecting 

 surfaces than transparent substances free from veins. He 

 finds that, with the exception of gold, in the case of the met- 

 als observed by him, the prime angle of incidence, or polari- 

 zation angle, diminishes with the diminution of the wave 

 length, the reverse of what takes place with transparent 



