One of the most essential points in a good micrometer is that all the webs shall be so nearly in the same plane as to be well in focus together under the highest powers used, and at the same time absolutely free from " fiddling."
The result is that no trace of " fiddling " exists, and the movable and fixed webs come sharply together in focus with the highest powers.
In order that a large part of the field of view may be in focus at once, it is desirable that the locus of the focused spectrum should be nearly perpendicular to the line of vision.
Diffraction when the Source of Light is not seen in Focus.
Savary dwells on the difficulty both of procuring lenses sufficiently equal in focus and of accurately adjusting and centring them.