274 PHYSICS. 



thickness of the film beiug at the center. To produce the rings a little 

 collodion is dropped on a surface of mercury. It is drawn out on all 

 sides into a thin iridescent film, which when hard may be floated oft' on 

 to paper. Mastic varnish on the surface of water gives similar films. 

 Drops of volatile mineral oil on mercury, or even the film of moisture 

 condensed from the breath, give rings which of course are transient. 

 At a meeting of the Physical Society of Paris, Guebhard projected these 

 films on the screen ; and showed that even the films condensed from the 

 breath may exhibit phoueidoscopic properties. The various vowels 

 being pronounced so that the breath impinges on the surface of the 

 cooled mercury, rings are obtained having certain forms more or less 

 strongly characteristic of their diiferent qualities of tone. — {Mature, xxi, 

 242, January, 1880.) 



Michelson has communicated to the 'New York Academy of Sciences 

 some interesting observations upon the diffraction and polarization 

 effects produced by passing light through a narrow slit. If a fine ad- 

 justable slit be narrowed down very greatly, the colored diffraction 

 fringes widen out until when the width of the slit is reduced to less than 

 one fiftieth of a millimeter the central space only is seen, and appears 

 of a faint bluish tint. Moreover the Ught so transmitted exhibits traces 

 of polarization when regarded through a Nicol i^rism. As the slit is 

 narrowed, the depth of the tint and the amount of polarization increase, 

 'until when the oi>eningis only one-thousandth of a millimeter the color 

 becomes of a deep violet and the light is completely polarized. Slits of 

 obsidian give the best results, because finer edges can be worked on it. 

 The plane of polarization is at right angles to the length of the slit. — 

 {Xature, xxii, 133, June, 1880.) 



Crova has published a note on the two forms of polarizing prisms, 

 those of Kieol and of Foucault. In the former the two halves are 

 cemented by Canada balsam; in the latter they are separated by a layer 

 of air. In both cases the bases are oblique to the axis of the prism and 

 this occasions a considerable loss of light both at entrance and emer- 

 gence; to which, in the Foucault prism, must be added that lost by the 

 internal reflections. Both Hofi'mann and Duboscq constructed Foucault 

 prisms with faces normal to the axis, but the loss of light internally re- 

 mained. Prazmowski has made a polarizing prism which is like a ]S^icol, 

 except that the halves are cemented with linseed oil, the faces being 

 normal. More spar is required and the manufacture is long and difficult. , 

 Almost no loss of light, however, takes place internally, the index of the 

 oil being 1.485, very near to the extraordinary index of spar, 1.483. 

 The field is increased to 31°, against 21° in the Nicol and 8° in the 

 Foucault. For photometric purposes especially, Crova regards this 

 new prism of great advantage.— (./. P%«., ix, 152, May, 1880.) 



Govi has illustrated the laws of circular polarization by an exceed- 

 ingly beautiful projection experiment, founded on the well-known fact 

 tha.t the spectrum obtained of the colors produced by the interference 



