Question no.1:
If your eyes are too close set or far apart for the intraocular distance to be adjusted properly, you will have to use your microscope as a monocular instrument (i.e. look through one eyepiece with one eye). If you do this, it is important to keep both eyes open in order to avoid eyestrain.
Question no.2:
Objective Lenses in a Microscope must be dust free in order to have accurate visions of the subject placed in the disc and not to affect your eye when looking through it.
Question no.3:
The illumination system of the standard optical microscope is designed to transmit light through a translucent object for viewing. ... To obtain optimal use of the microscope, it is important that the light from the source both covers the object and fills the entrance aperture of the objective of the microscope with light.
