The microscope optics is the usual three-lens system: A: Objective, B: Field lens (dia=25mm, f=45mm),
C: Eyepiece (dia=12mm, f=25mm)
Four objective lenses are included with this instrument. Two are missing components (A: the brass base and lens; D: the lens). See also this page.
This bull's-eye lens was probably used for illuminating opaque samples. It is missing the support shaft, but would have been mounted on the top of the box.

The lens is plano-convex, dia=25mm, f=45mm