This plan, it will be observed, was generically different jfrom that adopted
by the elder Herschel, whose aim was confined to the comparison of stars
divided into small sets of nearly equal brightness. While Sir John
Herschel was engaged in this project, and in the midst of others of
greater magnitude, he invented a photometer, by means of which he hoped
to arrive instrumentally at the same sort of results which heretofore he
and his predecessors had sought from estimations alone, though now with
far greater precision. The implement he devised was of rude construction,
such as could be arranged from the scanty resources of a distant colony,
but it was, in principle, sufficient for his purpose. Koughly described, it
consisted of a pole, a prism, a small lens of short focus, a few strings and a
graduated tape. With these materials properly arranged, he could obtain,
in the focus of the lens, a microscopic image of the moon, and this he
could view in any direction, and at any measured distance from the eye,
so that being brought into the same line of sight with any particular star,
he could alter the distance of the tiny image, until it and the star appeared
to be equally bright. In this way, the brightnesses of some sixty-nine stars
were compared with that of a Centauri, and the results were tabulated.
These relative light intensities, and the results of the sequence-observations
were then expressed in magnitude, in such a way and on such a scale as



General Historical Survey of Astrometry. vii

best to accord with the accepted magnitudes contained in the best catalogues
then extant.
