116 MICROSCOPIC ILLUSTRATIONS, 



Mr. Nicol's single- image calcareous prism, a more 

 extended field of view will be obtained; notwithstanding 

 which, the latter possesses many advantages over the 

 former, most especially that of its being colourless. 



In cases where the ingenious invention of Mr. Nicol 

 cannot be procured, a rhomb of Iceland spar may be 

 well enough substituted, provided only that it has a 

 small aperture at the under surface, and that the upper 

 one is furnished with a sliding plate, with a correspond- 

 ing aperture, so that one or other of the images can be 

 seen at pleasure. Sir D. Brewster, who adopts this 

 method, cements a thin plate of glass upon both surfaces 

 of the rhomb, to take away their imperfections, and to 

 prevent accidents occurring, or deterioration of the 

 polish by cleaning. 



The peculiar advantages resulting from the application 

 of the rhomb of Iceland spar, I shall give in Sir David's 

 own words. "These rhombs maybe made even out of 

 rhombs crossed with veins, which multiply the images, 

 because the multiplied images are at too great a distance 

 from the principal ones to be visible. This is a pecu- 

 liar advantage, as it is often very difficult to get good 

 pieces of spar free from this composite structure. 



"This method of constructing a polarizing rhomb 

 enables us to take advantage of the two lateral images 

 which accompany the two principal images in crystals 

 crossed by on evein. These lateral images, shewn at 

 m, w, fig. 26, are distant from one another, and from the 

 principal images, b, c; and as each of them consists of 



