66 BENJ. PIKE S, .JR., DESCRIPTIVE CATALOGUE. 



Of the Index Glass. Upon the index, and near its axis, 

 is fixed a plain speculum, or mirror of glass, quicksilvered. 

 It is set in a brass frame, and is placed so that the face of 



Fig. 93. 



A. Frame and arc ; B. Index : C. Nonius scale ; D Index glass ; E. Dark glass 

 er screens; F. Horizon glass ; G. Vane or sight 



