16 E. M. NELSON ON THE GENERAL CHARACTER OF 



This consists of a converging and diverging meniscns. The 

 difference between these two last combinations is at once shown 

 by the different characters of the reflections from their 

 cemented surfaces. The intelligent observer will note that the 

 concave exterior surface when seen through the convex surface 

 has its sign changed from e to i ; of this I spoke above. 



An unequal biconcave doublet : — 



Most concave side, li (se) si. 



Least concave side, Li (si) se. 



It consists therefore of a biconcave and a converging 

 meniscus. The si in the most concave side is not what one 

 would have expected. The interior curve of the meniscus 

 must have a short radius, so that it overpowers the exterior 

 concave surface of the biconcave. The least concave side is 

 very flat, as shown by L. 



A biconvex triple, equiconvex : — 



e (i) (e) i both sides alike. 



It consists of a biconvex, binconcave, biconvex. A plano- 

 convex triple consisting of a biconvex, a biconcave, and a plano- 

 convex, will have the same form, only the plane side will be 

 Le instead of se and li instead of si. 



An unequal biconvex triple as in a properly constructed opera 

 glass objective : — 



Most convex side, le (si) (Li) li. 



Least convex side, le (Le) (se) si. 



The lenses are therefore biconvex, diverging, and converging 

 menisci. 



A biconcave triple as in a properly constructed opera glass 

 eye-piece : — 



From side of thickest lens, li (se) (le) se. 



From side of thinnest lens, li (Le) (li) se. 



Its form is biconcave, converging meniscus, and diverging 

 meniscus. 



If the middle were a piece of plain glass, which is sometimes 

 fraudulently put, its form would be : — 



i (e) (e) e, 

 which would read the same from both sides. But if it had a 

 thin converging meniscus in the middle, it would read : — 



li (le) (le) se from one side. 



li (Le) (Le) se from the other. 



