Astronomical Socicti/, '305 



ever such simultaneous points there may be, they must all lie in the 

 same plane ; and hence, if the second point which must exist could 

 be determined, then the great circle in the plane of which the axis of 

 the magnet itself is situated would be determined 3 and thus another 

 test would be afforded of the truth or error of the hypothesis itself. 

 Mr. Davies suggests that as this plane will be symmetrical with respect 

 to the phaenomena taking place on each side ot* it, its position might 

 be tentatively assigned from a series of observations of those phaeno- 

 mena, especially of the dip and intensity ; the variation being for ob- 

 vious geometrical reasons excluded. 



Though the resulting formula does not in its literal form appear to 

 be capable of decomposition into factors, yet from some considera- 

 tions, chiefly analogical, Mr. Davies is led to hazard the conjecture 

 that it is capable of such decomposition ; but as this is uncertain, he 

 builds no consequences upon it, but leaves those consequences which 

 would flow from it, open till it shall be discovered whether they would 

 be justified by the conjecture itself being proved to be correct. 



A paper was also read, entitled, ** On certain Peculiarities in the 

 double Refraction, and Absorption of Light, exhibited in the Oxalate of 

 Chromium and Potash." By Sir David Brewster, K.H.,L.L.D., F.R.S. 



The crystals of the oxalate of chromium and potash are, generally 

 speaking, opake; for at thicknesses not much greater than the 25th 

 of an inch, they are absolutely impervious to the sun's rays, and their 

 colour, seen by reflected light, is nearly black j but when powdered, 

 they are green ; and the colour of the smaller crystals, viewed either 

 by reflected or by transmitted daylight, is blue. One of the most 

 remarkable of the properties of this salt is the difference of colour in 

 the two images formed by double refraction. At a certain small thick- 

 ness, the least refracted image is bright blue, and the most refracted 

 image bright green. The blue is found by analysis with the prism to 

 contain an admixture of green, and the green an admixture of red ; 

 and by candlelight this red predominating over the green, gives the 

 crystal a pink hue. At greater thicknesses the blue becomes purer 

 and fainter, and the green passes into red ; and at a certain thickness 

 the least refracted blue image disappears altogether, and the most 

 refracted image is alone seen. At still greater thicknesses this image 

 also disappears, and absolute opacity ensues. When the crystal is 

 exposed to polarized light, with its axis in the plane of polarization, 

 the transmitted light is green ; but when the axis is perpendicular to 

 that plane, the transmitted light is blue. A solution of the salt ex- 

 hibits the same general action upon light as the solid, with the excep- 

 tion of double refraction. This salt has also the peculiar property of 

 exciting a specific action upon a definite red ray, situated near the 

 extremity of the red portion of the spectrum* 



ASTRONOMICAL SOCIETY. 



1834. Dec. 12. — ^The President read a letter from His Royal High- 

 ness the Duke of Sussex, acknowledging the vote of thanks passed 



* See our Number for January, p. 134. 

 Third Scries. Vol.6. No. 34. April 1835. 2R 



