The assembled eyepiece has this aperture.
The microscope shows this Huygenian eyepiece consisting of two plano-convex lenses arranged with the flat side facing the observer. The aperture in the middle limits spherical aberration. Eye lens dia=1.2cm f=2.1cm; field lens dia=2.2cm, f=4.2cm
There is only one objective remaining with the Student Microscope. It is a 3/4 inch lens
Lenses are attached to the body tube via this threaded collar.