Parallel optical
system is probably a different way of saying "Common main objective"
system. Common main objective stereomicroscope designs center on the refracting
action of a single, large diameter objective lens, through which both the left
and right channels view the object. Each channel operates as an independent
optical train parallel to the other (this is the reason they are also known as
parallel microscopes.
originally found here :- http://www.lab360.co.in/microscopes-telescopes.htm
