Dec. 1 8. 1873] 



NA TURE 



127 



POLARISATION OF LIGHT 



I, I 



LIGHT is said to be polarised when it presents cer- 

 tain peculiarities, hereafter to be described, which 

 it is not generally found to possess. These peculiarities, 

 although ^'ery varied in their manifestations, have one 

 feature in common, viz. that they cannot be detected by 

 the unassisted eye ; consequently, special instrumental 

 means are required for their investigation. 



The origin and meaning of the term polarisation will 

 be better understood when some of the phenomena have 

 been witnessed or described, than beforehand, and I 

 therefore postpone, for the present, an explanation of it. 



The subject of polarisation may be approached by 

 either of two roads, the experimental or the theoretical. 

 The theoretical method, which proceeds upon the prin- 

 ciples of the Wave Theory of Light, is remarkably com- 

 plete and explicit ; so much so that it not only connects 



Fig. 3. 



Fic. 4. 



together many very diversified phenomena, but even, in 

 some cases, has suggested actual prediction. But inas- 

 much as the theory without experimental facts would be 

 little better than a study of harmony without practical 

 music, it will be best to begin with experiment. 



It was stated above that certain instrumental means 

 were requisite for detecting polarisation. Now there are 

 various processes, some occurring in the ordinar)' course 

 of natural phenomena, others due to instrumental appli- 



ances, whereby a ray of light may be brought into the 

 condition in question, " or polarised." And it is a 

 fact both curious in itself and important in its applica- 

 tions, that any one of these processes (not necessarily 

 the same as that used for polarising) may be used 

 also as a means of examining whether the ray be in that 

 condition or not. This latter process is called " analys- 

 ation." When two instruments, whether of the same 

 or of different kinds are used, they are called respectively 



the " polariser " and the " analyser ;" and the two together 

 are included under the general name of " polariscope." 



The four principal processes by means of which a ray 

 of light may be polarised are, reflexion, ordinary refrac- 

 tion, double refraction, and scattering by small particles. 

 These methods will be considered in order ; but before 

 doing so, it will be convenient to describe the phenomena 

 of polarisation as exhibited by some instrument tolerably 

 simple in its action and of easy manipulation. For such 

 a puipose a plate of crystal called tourmalin will perhaps 

 serve better than any other to begin with. 



Tourmalin is a crystal of which there are several varie- 



ties, differing only in colour. Very dark specimens gene- 

 rally answer the purpose well, excepting that it is difficult 

 to cut them thin enough to transmit much light. Red, 

 brown, or green specimens are usually employed ; the 

 blue are for the most part optically unsuitable. Some 

 white, or nearly white, specimens are very good, and may 

 be cut into thicker plates without loss of light. 



If we take a plate of tourmalin cut parallel to a parti- 

 cular direction within the crystal called the optic axis (the 

 nature and properties of which will be more particularly ex- 

 plained hereafter), and interpose it in the path of a beam of 

 light at right angles to the direction of the beam, the 



only effect perceptible to the unassisted eye will be a 

 slight colouring of the light after transmission, in conse- 

 quence of the natural tint of the particular piece of 

 crystal. But if we examine the transmitted beam by a 

 second similar plate of tourmalin placed parallel to the 

 former, the following efi'ects will be observed. When the 

 two plates are similarly placed, i.e. as if they formed one 

 and the same block of crystal, or as it is technically 

 expressed, with their optic axes parallel, we shall per- 

 ceive only, as before, the colouring of the light due. to 

 the tints of the two plates. But if either of the plates be 

 then turned round in its own plane, so as always to 



