Intelligence and Miscellaneous Articles. 391 



relative to the rays of the principal tints of the spectrum ; we know 

 consequently the inclinations of the axis of the thin plate to each of 

 these planes ; we calculate the corresponding intensities of the emer- 

 ging light by means of one of Fresnel's formulae for thin crystalline 

 plates. Lastly, we apply to these values Newton's rule, reduced to 

 formulae by M. Biot, which allows us to determine the nature of the 

 tint and its comparative intensity. 



To avoid calculating each observation, it will be convenient to 

 construct double entry Tables, containing the cyanometric states 

 corresponding with the azimuth observed. 



The mode of determining the origin of the angles is very simple. 

 Before operating, the screen is removed, to allow a direct passage to 

 the Ught ; the plates of quartz are removed, and the azimuth cor- 

 responding with the extinction of the polarized ray of the system 

 reproducing the tint is noted, pointing it towards a brightly luminous 

 part of the sky ; this is easily obtained in a minute or two. The 

 thin plate is then replaced and turned by means of the alidade until 

 the extinction of the light again takes place ; in this position the 

 axis of the plate is parallel to the principal section of the polarizer. 



In order that the tint of the sky may not be altered, care must be 

 taken to direct the principal section of the polarizer of tlie other 

 system in the plane of polarization of the incident light ; for this 

 purpose, the polariscope with double rotation is employed, as I have 

 already 'indicated in describing the polarimeter.— Com^^/es Rendus, 

 November 17, 1856, p. 982. 



COMPARATIVE EXAMINATION AND ANALYSIS OF EUDIALITE AND 

 EUKOLITE. BY A. DAMOUR. 



Eudialite, a mineral which receives its name from the facility 

 with which it may be dissolved by acids, has only been observed 

 hitherto in a single locality, Kangerdluarsuk, on the western coast 

 of Greenland, where it was discovered fourteen years ago by Dr. 

 Giesecke. The numerous specimens which have been since collected 

 in the same locality, exhibit this substance associated with sodahte 

 and Arfvedsonite. 



More recently a mineral has been found in the zirconian syenite 

 of Brevig, in Norway, the physical and chemical characters of which 

 have much analogy with those of eudialite. This new mineral was 

 first of all classed as a distinct species under the name of eukohte. 

 Subsequently Scheerer analysed and thought that it might be 

 referred to Wohlerite. The resemblance between eudialite and 

 eukolite leading to a presumption that they might have the same 

 composition, the author has submitted them to a comparative 



Eudialite forms crystals derived from a rhombohedron of 73° 30'; 

 it occurs also in crystaUine masses, sometimes exhibiting a tolerably 

 distinct double cleavage which leads to the hexagonal prism of 120 . 

 Planes of cleavage parallel to the faces of the primitive rhomhohe- 

 dron and to those of a more obtuse rhombohedron, have also been 

 mentioned. Its colour is rose or violet-red, presenting tints identical 

 with those of different varieties of almaudine garnet. Eudiahte is 



