Huygenian eyepiece

noun

Huy·​ge·​ni·​an eyepiece ˌhī-ˈgē-nē-ən-  ˌhȯi- 

-ˈgen-ē-

variants also Huygens eyepiece

ˈhī-gənz-  ˈhȯi- 

: a compound eyepiece used especially with achromatic objectives that consists of two plano-convex lenses separated by a diaphragm with the convex sides facing the objective