74 Scientific Intelligence. [Jan. 



Whether this has any connection with the substance in question^ 

 time will determine. 



III. New Properties of Light. 



Dr. Brewster has favoured me with the following account of his 

 new and curious experiments on the polarization of light: — 



" 1 have found that light transmitted obliquely through all todies, 

 whether crystallized or uncrystallized, suffers polarization like one 

 of the pencils produced by double refraction ; and from a great 

 number of experiments, I have been enabled to determine the law 

 by which all the phenomena are regulated. 



" If light is incident at any angle, except a right angle, upon the 

 surface of a transparent body, a portion of the transmitted pencil 

 will suffer polarization. The quantity of polarized light varies as 

 the cotangent of the angle of incidence ; and there is always a 

 particular angle, depending on the refractive power of the body, at 

 which the emergent light is wholly polarized. When the light is 

 transmitted necessarily through several parallel plates, either in 

 contact or at a distance, the cotangents of the angles of polarization 

 are always to one another as the number of plates employed ; and 

 the number of plates multiplied by the tangent of the angle at 

 which they polarize light is a constant quantity. If the angle of 

 incidence exceeds the angle of polarization, the pencil will still 

 emerge in a polarized state. 



" A parcel of 8 plates of plate glass polarizes the transmitted 

 light at an angle of 79*^ H'^ and at any angle of incidence greater 

 than this. 



" A parcel of 16 plates polarizes the light at any angle above 

 69° 4', and 



*' A parcel of 47 plates at any angle above 41° 41'. 



*' Similar effects, varying however with the refractive power, are 

 produced by plates of mica, by films of blown glass, by coats of 

 grease, gold-beaters' skin, and even gold leaf itself. 



" Malus's discovery of the polarization of light by reflection is, 

 perhaps, one of the most brilliant discoveries that optics has ever 

 received ; but though it developed a new set of phenomena ana- 

 logous to those produced by doubly refracting crystals, yet as the 

 polarization was obviously effected by reflection, and not by refrac- 

 tion, it did not furnish any information respecting the methods 

 by which these crystals polarized the transmitted light. Tiie disco- 

 very, however, of the polarization of light by oblique refraction 

 forms the connecting link between these two classes of phenomena, 

 and holds out the prospect of a direct explanation of the leading 

 phenomena of double refraction, of the polarizing power of the 

 agate, and of the partial polarization of light by polished metals."* 



* From some things contained in (he preceding statement, it is evident that Dr. 

 Brewster is unacquainted with the fact tliat Mains nearly three years ago discovered 



