produces in the Double Befraction of Crystals. 4-13 



the deviation, for the density of the surrounding air bad be- 

 come less. It is easy to calculate bow much this augmenta- 

 tion would have been. The index of the ray F bemg at the 

 ordinary temperature = 1-66802, the deviation A calculated 

 by the formula sin i (A +a) = 1 "000051 . + 1-66802. sin i s 

 becomes = 52° S^'^li", or greater by Si" than 52° 53' 43". 



000051 ^ -„o co/_iW/ 

 From these 31" we must subtract ^.^^^^^.^ tang 52 53 _ 14 , 



or the angle which the ray deviates at the plate of mica, so 

 that there' will remain only IT". Though this quantity is the 

 least which I could directly measure with the repeating circle, 

 I have no reason to believe that if it did occur, it would 

 have escaped me. Whence we may conclude, that thc_ re- 

 fractive jw-wer of calcaremis spar for the ordinary ray, cithn- 

 docs not change at all loith the temperature, or decreases xmth it 

 by a quantity extremely small. _ 



b. The Extraordinary Spectr?im.— The deviation ot the ray 

 F was found to be augmented by a difference of temperature 

 of 64°, a quantity — 2' 28". 



If we add to this the correction for the passage of the light 



„ . 0-000051 o o ,„, nni, 



through the plate of mica = y^qqqJI tang ^6 18 Jb 



= S"-0, the total augmentation produced by the temperature in 

 the deviation of the extraordinary ray becomes 



= 2' 34." 

 The calculation gives the index = 1-49118, whereas ^vt the 

 ordinary temperature it was = 1-49075. Hence it follows, 

 that a difference of temperature of 64° lyroduces in the index oj 

 refraction of the extraordinary ray of calcareous spar, an in- 

 crease of + 0-00043. , ^_^ „ . 



Durin.r my stay at Berlin in the month ot May, of the pie- 

 sent vear(1832),I had fortunately an opportunity of conhrni- 

 in-, at' the house of M. Mitscherlich, and in his presence, this 

 reu arkable property of calcareous spar -that the deviation ot 

 the ordinary ray does not change, at least not ni an appreci- 

 able manneJ, with the temperature; while, on the contra, y, that 

 of the extraordinary ray increases considerably with the tcni- 

 p.Mature. This property is in a certain manner connected 

 lith the discovery of M. Mitscherhch,-lhat c-«/a/;ro«. spai, 

 u-hcn the temperature rises, dilates UselJ in the direction of the 

 avi-^ of ayslallizalion, but undergoes in a direction perpen- 

 dicular to the axis a conlraclion extremely small. 1 he crystal 

 thus approaches to a cube with an increase ol tempera urc, 

 and the double refraction ought, conse(,uently, to dimmish, as 



