582 SPECIAL METHODS OF DISCOVERY. 



general relation between their optical properties and crys- 

 talline form ; a correspondence between the degrees of 

 symmetry of the crystalline shape and the optical pheno- 

 mena; that those crystals which are uni-axial in their 

 optical properties and give circular rings by polarised light 

 are also uni-axial in a crystallographic sense ; and that those 

 which are of other forms are usually bi-axial in optical 

 properties. By comparing together also those crystalline 

 substances which twist a beam of polarised light with 

 those which crystallise in peculiar modified forms, such as 

 crystals of quartz with plagihedral faces at both ends, 

 Sir J. Herschel discovered that the right-handed or left- 

 handed optical property of crystals correspond with what 

 may be termed their right-handed or left-handed crys- 

 talline form. 



Sir Edward Sabine, by comparing the observations of 

 Lamont of Munich, that the diurnal variation of the mag- 

 netic needle increased during five and a half years, and 

 then decreased during a similar period, with that of Schwabe 

 of Dessau, that the increase and decrease of spots on the 

 sun were coincident with and occupied the same period as 

 those changes, discovered that the two phenomena were 

 closely related to each other. Magnetic storms were dis- 

 covered soon after the year 1820, and, by a comparison of 

 the observation of M. Kupffer at Kasan in Eussia with 

 those of M. Arago at Paris, about the year 1825, the 

 discovery was made that those storms occurred simul- 

 taneously over large tracts of the earth. It was by 

 comparing stars, and classing them into groups in accord- 

 ance with their relative degrees of brightness, that W. 

 Herschel discovered no less than 500 double or binary 

 stars ; a few, however, were known previously. He first 

 discovered, by comparison of their relative positions, that 

 they revolve round each other. 



