Double Image Micrometers are described in the article HELIO METER (q.v.).
P. io) claim that became universal; the methods of illumination were improved; and micrometers with screws of previously unheard of fineness and accuracy were produced.
Micrometers used for subdividing the spaces on graduated circles and scales have, in general, only a single pair of cross-webs or parallel webs moved by a single screw.
We have not had an opportunity of testing this, nor Grubb's more recent models; but, should it be found possible to produce such images satisfactorily, without distortion and with an apparatus convenient and rigid in form, such micrometers may possibly supersede the filar micrometer.