The least count of a $$200$$ $$mm$$ long spirit level is given as $$10$$ seconds per division. The gradient of the spirit level expressed as $$mm$$ over one meter is _____________.

The diameter of a hole is given as $${50^{\matrix{ { + 0.15} \cr { + 0.00} \cr } }}\,\,mm.$$ The upper limit on the dimension in $$mm,$$ of the shaft for achieving maximum interference of $$50micron$$ is .................

Abbe's principle of alignment is followed in: