Elementary Nomenclature in Geometrical Optics. 

 For a plano-convex lens, as in fig. 2, 



Convergivity = (p — 1) - ; ^ = 0. 



Fig. 1. 



Fig. 2. 



o' 



0A= r; 0'A'=/ in all the figures. 

 For a concavo-convex lens, as in fig. 3, 



Convergivity =(fM-l)(-- -/J ; r' > 

 Fig. 3. 



401 



o r 



o urmr- 



And similar formulas for the three diverging lenses. 



Fig. 4. 



(3') The convergence of the pencil of light in fig. 4, and 

 the divergence of the pencil of light in fig. 5 = 1/SO. 

 Phil. Mag. S. 5. Vol. 28. No. 174. Nov. 1889. 2 G 



