This form of micrometer is of course capable of giving results of high precision, but the drawback is that the process involves a minimum of six pointings and the entering of six screw-head readings in order to measure the two co-ordinates of the star.
At the Cape of Good Hope, after more than 200,000 pointings had been made, the screw-errors were redetermined; the results proved the truth of the above conclusions, viz.
The excellent manner in which the scales and micrometers are mounted, the employment of a compound microscope for viewing the scales, with its ingeniously arranged and admirably efficient reversing prism, and the perfection of its slow motions for focusing and reading, combine to render this a most accurate and convenient instrument for very refined measures, although too slow for work in which the measures must depend on single pointings in each of two reversed positions of the plate, and where speed of working is essential.
Elkin found that the chief drawbacks to speed and convenience in working this heliometer were: (I) The loss of time involved in entering the corresponding readings of the micrometer pointings on two scales.
(2) That an additional motion intermediate between the quick and slow motion in position angle was necessary, because, whilst the slow motion provided by Repsolds was admirably adapted for adjusting the pointings in position angle, it was too slow for causing the images to "cross through " each other in the process of measuring distances.