1818.] 



Register of the Weather in Iceland. 



175 



(T/ic Supplementary Register will appear in the next Number.) 



Article IV. 



Description of a Method of making Douhly Refracting Prisms 

 p e rfectly Achromatic. By David Brewster, LL.D. F.R.S. 

 Loudon and Edinburgh. 



In the course of my experiments on the polarisation of light, 

 1 have long been in the habit of making use of perfectly achro- 

 matic prisms of calcareous spar ; but until I ascertained from 

 M. Biot that the method of constructing such prisms had neither 

 occurred to himself, nor, as far as he knew, to any other person, 

 I did uot think of laying it before the public. 



When we form a prism of calcareous spar for the purpose of 

 analyzing light that has been submitted to the action of crystal- 

 lized bodies, and correct the colour of one of the images by the 

 equal and opposite dispersion of a prism of glass, we shall always 

 find that a considerable portion of the colour of the other image 



