Indeed, in those days, the difficulties attached to such measures, and to the measurement of distances with the filar micrometer, were exceedingly great, and must have taxed to the Utmost the skill and patience of the observer.
The Modern Filar Micrometer.
When equatorial mountings for telescopes became more general, no filar micrometer was considered complete which was not fitted with a position circle.'
The use of the spider line or filar micrometer 1 In 1782 (Phil.
A still further facility was given to the use of the filar micrometer by the introduction of clockwork, which caused the telescope automatically to follow the diurnal motion of a star, and left the observer's hands entirely at liberty.'
The latter are in fact little microscopes carrying a vernier etched on glass, in lieu of a filar micrometer.
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 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.
To Bouguer in 1748 is due the true conception of measurement by double image without the auxiliary aid of a filar micrometer, viz.
He, however, successfully employed the instrument in measuring double stars, so close as I" or 2", and using a power of 300 diameters, with results that agreed satisfactorily amongst themselves and with those obtained with the filar micrometer.
The other telescope is corrected for visual rays and its image is formed on the plane of the spider-lines of a filar micrometer.