Bell. — Opacity of Certain Glasses. 



Plate I. 



A.O. Crown. 

 Ct.'l mm. 



Medium smoke. 

 ."j.G mm. 



B. andL. Crown. 

 4.2 mm. 



Amethyst glass. 

 2.8 mm. 



Bare tube. 

 15 seconds. 



Proc. Amer. Acad. Arts and Sciences. Vol. XLVI. 



