136 M. Rudberg on the Refraction of the differ ently- Coloured 



sets of reduced observations, which are of the highest value in 

 reference to this important branch of meteorology. These ob- 

 servations will be published in successive Numbers of this 

 Journal ; and while they will enable me to compare my own 

 theoretical view with observations, they will be received by the 

 scientific meteorologist as data of inestimable value in fixing the 

 principles of this new science. 



It appears from the first of the preceding tables, that the 

 mean temperature of Nicolaieff for four successive years, from 

 1827—1830, at 10" a.m. and 10'' p.m. is T-o'2, Reaumur, or 

 48°*92 Fahr. When we correct this result by + 0-122, the 

 quantity by which the mean of 10'' and 1 0'' differs from that of 

 the 24' hours, we obtain, Fahr. 



Corrected mean temp, of Nicolaieff" 49°-04' 



Add for elevation of 20 toises -36 



Mean temp, of level of sea 49°'4!0 



Mean temp, calculated by formula T = 86-3 sin. 

 D — 3~\ D, the dist. from the Asiatic Pole being 

 = 39°-27 51-33 



Difference between the obsei'vation and the formula +1°'93 

 The mean temperature of the year 1827 at Nico- 

 laieff" was fully 52°A 



So that the formula gives a result within the varying limits 

 of observations for different years, and diff"ering very little 

 from the mean result. D. B.] 



XXXI. On the Refraction of differeiitly-colmired Rays in 

 Crystals, with one and two Axes of Double Refraction. By 

 M. Rudberg. 



[Continued from page 6.] 



Section II. Refraction in Ciystals 'with two Optical Axes: 



''I "'HE crystals of this kind, which I have been able to pro- 

 -*• cure, were arragonite, colourless topaz, and the topaz of 

 Schneckenstein. I have not, however, been able to make use 

 of the last of these, of which I have large and fine specimens, 

 because throughout their interior there are cleavage planes, 

 which being always parallel to the external planes reflect the 

 sun's rays in so confused a manner, that the spectrum is 

 not distinct. I have consequently been able to make experi- 

 ments only with arragonite, and white or colourless topaz. 



Before giving an account of these experiments, I shall briefly" 

 explain the theory of double refraction in crystals with two 



