William Gascoigne's invention of the filar micrometer and of the adaptation of telescopes to graduated instruments remained submerged for a quarter of a century in consequence of his untimely death at Marston Moor (1644).
The Modern Filar Micrometer.
No auxiliary filar micrometer was required, as in Savary's heliometer, to measure the interval between the limbs of two adjacent images of the sun, it being only necessary to turn the screw with the divided head to change the distance between the object-glasses till the two images of the sun are in contact as in fig.
The other telescope is corrected for visual rays and its image is formed on the plane of the spider-lines of a filar micrometer.
The method originally used by Huggins, who first conceived and proved the possibility of measuring stellar velocities in the line of sight, was to measure with a filar micrometer the displacement of some well-known line in a stellar spectrum relative to the corresponding line of a terrestrial spectrum.
The use of the spider line or filar micrometer 1 In 1782 (Phil.
Dawes very successfully used this micrometer in conjunction with a filar micrometer, and found that the precision of the measures was in this way greatly increased (Monthly Notices, vol.
To Bouguer in 1748 is due the true conception of measurement by double image without the auxiliary aid of a filar micrometer, viz.
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.
The latter are in fact little microscopes carrying a vernier etched on glass, in lieu of a filar micrometer.