The compound microscope uses
several lenses to produce a highly magnified image of an object. The diagram shows the microscope in normal
adjustment, that is, with the final image at the near point (25 cm from the eye) (distance D
from the eye lens). This setting gives the maximum angular size of image without eye
strain.

The objective lens produces a real, inverted image of the object 0 at I_{1} This is
then viewed by the eye lens and this gives a final virtual image at I.

where f_{e} is the focal length of the eye lens, f_{o}
that of the objective lens and v the distance of I_{1} from the objective lens.
The
resolving power of a microscope depends on the refractive index of the medium above the slide (n) and the
angle subtended by the object at the objective (a)