PHILOSOPHICAL SOCIETY OF WASHINGTON. 127 



Just below it is another right-angled prism, of the same dimen- 



TTr. r' '^' '''''''' ^^ ^^'^ P"^"^ '•« indicated by 



daik shading m the diagram. The right angles of both prisms 



are truncated, and the facets are cemented together in such a 

 manner that the long sides of the prisms are paLlel. The br ss 

 pnsn. shps transversely in a groove in the top of a holder O 

 which .s fitted into the substage of the microscope. DDi 

 1 y 



IS a 



