The determination of the distances of the fixed stars from the small
apparent shift of their positions when viewed from widely separated
positions of the earth in its orbit was one of the most refined
operations of the observatory. The great precision with which this
minute angular quantity, a fraction of a second only, had to be
measured, was so delicate an operation with the ordinary micrometer,
though, indeed, it was with this instrument that the classical
observations of Sir Robert Ball were made, that a special instrument,
in which the measures were made by moving the two halves of a divided
object glass, known as a heliometer, had been pressed into this
service, and quite recently, in the skillful hands of Dr. Gill and Dr.
Elkin, had largely increased our knowledge in this direction. It was
obvious that photography might be here of great service, if we could
rely upon measurements of photographs of the same stars taken at
suitable intervals of time. Professor Pritchard, to whom was due the
honor of having opened this new path, aided by his assistants, had
proved by elaborate investigations that measures for parallax might be
safely made upon photographic plates, with, of course, the advantages
of leisure and repetition; and he had already by this method
determined the parallax for twenty-one stars with an accuracy not
inferior to that of values previously obtained by purely astronomical
methods.