POLARISED LIGHT. 



107 



each other, the ordinary ray refracted in the first will be the extraordi- 

 nary in the second, and so on vice versd. At the intermediate position 

 of the two crystals there is a subdivision of each ray, and therefore 

 four images are seen ; when the crystals are at an angle of 45 to each 

 other, then the images are all seen of equal intensity. 



Mr. Nicol first succeeded in making rhombs of Iceland spar into 

 single-image prisms, by dividing one into two equal portions. His 

 mode of proceeding is thus described in the Edinburgh Philosophical 

 Journal (vol. vi. p. 83) : " A. rhomb of Iceland spar of one-fourth of an 

 inch in length, and about four-eighths of an inch in breadth and thick- 

 ness, is divided into two equal portions in a plane, passing through the 

 acute lateral angle, and nearly touching the obtuse solid angle. The 

 sectional plane of each of these halves must be carefully polished, and 

 the portions cemented firmly with Canada balsam, so as to form a 

 rhomb similar to what it was before its division ; by this management 

 the ordinary and extraordinary rays are so separated that only one of 

 them is transmitted : the cause of this great divergence of the rays is 

 considered to be owing to the action of the Canada balsam, the refrac- 

 tive index of which (1-549) is that between the ordinary (1-6543) and 

 the extraordinary (1*4833) refraction, of calcareous spar, and which will 

 change the direction of both rays in. an opposite manner before they 

 enter the posterior half of the combi- 

 nation." The direction of rays passing 

 through such a prism is indicated by 

 the arrow, fig. 72, and the combination 

 is shown mounted, one for use under fi g> 72. 



the stage of the microscope, fig. 73, 



termed the polariser; another, fig. 74, screwed on to and above the 

 object-glasses, is called the analyser. 



73. 



fig. 74. 



