POLARISED LIGfiT. 137 



"A rhomb of Iceland spar of one-fourth of an inch in 

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

 thickness, 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 refractive 

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

 (1-6543) and the extraordinary (1-4833) refraction of 

 calcareous spar, arid which will change the direction of 

 both rays in an opposite manner before they enter the 

 posterior half of the combination." The direction of rays 



Fig. 83. 



passing through such a prism is indicated by the arrow, 

 fig. 83, and the combination is shown mounted, one for 



Fig. 85. 



use under the stage of the microscope, fig. 84, termed the 

 polariser; another, fig. 85, screwed on to and above the 



