8 LECTURES TO SCIENCE TEACHERS. 



We are not however here so much specially concerned with 

 the fact of double refraction as with a peculiarity which 

 accompanies it, viz. that each of the rays produced by double 

 refraction is itself polarised, and that the two are polarised 

 in planes at right angles to one another. This is seen by 

 analysing two such beams of light. If the analyser be placed 

 so as to extinguish one of them, the other is at its brightest. 

 If the analyser be then turned round the first beam will begin 

 to appear, while the second fades. When the angle of turn- 

 ing reaches 45, the two are equally bright ; and when it 

 reaches 90, that which has been extinguished is at its bright- 

 est, while the other has disappeared. An arrangement of 

 spar producing these effects with the greatest degree of 

 separation between the two sets of rays, is called a double- 

 image prism ; and from its convenience in turning round is 

 in many ways preferable to the glass plates. But when great 

 size is required, it is both difficult and expensive to obtain 

 suitable spar for the construction of the instrument. 



It is, however, not always necessary, nor indeed always 

 convenient, to have in the field of view both the images due 

 to double refraction. We often require a simple beam of 

 polarised light, and in such case a double image prism 

 gives us more than we want. To meet this inconvenience, a 

 very ingenious arrangement was devised for getting rid of 

 one of the beams. A block of spar so prepared as to give 

 good double refraction is cut diagonally across, and cemented 

 together again with Canada balsam. The refractive power of 

 this substance is different from that of the spar : and the 

 angle of cutting is so arranged that at the surface of the 

 balsam one of the rays undergoes total reflexion, and is thus 

 thrown entirely out of the field. There then remains visible 

 only a single beam of perfectly polarised light. This instru- 

 ment is named after its inventor, a Nicol prism. The end 

 faces of such a prism are very delicate, and should be pre- 

 served and cleaned, when necessary, with the greatest care. 

 A pair of instruments of whatever kind, one of which is 

 used for polarising, the other for analysing, light, is called a 

 polariscope ; and a pair of Nicol prisms, or a JSTicol and a double 

 image prism, is the most convenient form of polariscope as 

 yet invented. It is not, however, essential that the polariser 

 and analyser should be instruments of the same kind ; we 

 may polarise by reflexion or refraction with glass plates, and 



