186 



Transactions of the 



FMontlily Microscopical 

 L Juunial, April 1, 1870. 



IV. — A New Method of using Barker's Films. 

 By Edwakd Eichards. 



(Bead lefore the Eoyal Microscopical Society, March 9, 1870.) 



I AM desirous of bringing before the notice of the Society a new 

 and more exact method of working with Darter's Films of 

 Selenite. 



The instrument consists of three cells of brass cut out on the 

 edge so as to leave eight projections, two of which resemble the end 

 of an arrow denoting the positive axis, two semicircular at right 

 angles denoting 90°, and four smaller which stand for 45°, which 

 as signs cannot be mistaken ; and are rotated in three separate 

 arms while under a polarizing microscope ; and if the Selenite films 

 are turned round till their positive axes coincide they give the sum 

 of their combined retardation. 



If any be turned till their positive axes are at 90°, or the semi- 

 circular projections to the positive axis or arrow-head of the others, 

 the lesser number is subtracted from the greater. 



For instance when the positive axis of the fths is placed at right 

 angles to the positive axis of the fths, the sum of the difference only 

 is obtained = fths. 



If the Jth is now added with its positive axis coinciding with 

 the positive axis of the fths, seven quarters are obtained ; but if 

 placed to coincide with the positive axis of the fths, five quarters is 

 the result. 



Therefore, by subtracting by the semicircular projection or 



