It is the most striking example of how fully HERSCHEL realized that the eye of the observer is a material part of the optical apparatus of astronomy. Simple as this principle may appear, it was an absolute novelty in his day.
"I took care to bend my head so as to receive the picture of the belt in the same direction as I did formerly. This was a precaution that occurred to me, as there was a possibility that the vertical diameter of the retina might be more or less sensitive than the horizontal one."
Astronomers will recognize in this the first suggestion of the processes which have led to important results in the hands of Dr. OTTO STRUVE and others in the comparison of the measures of double stars by different observers, each of whom has a personal habit of observation, which, if not corrected, may affect his results in the way which HERSCHEL was striving to avoid.
_Researches on the Relative Brightness of the Stars: Variable Stars._
No research of HERSCHEL'S was more laborious than the elaborate classification of the stars according to their comparative brightness, which he executed during the years 1796 to 1799. It was directly in the line of his main work--to find out the construction of the heavens.
His first paper had been upon the variable star _Mira Ceti_. Here was a sun, shining by its native brightness, which waxed and waned like the moon itself. This star is periodic. It is for a long period invisible to the unassisted eye. Then it can just be seen, and increases in brightness for a little over a month, and attains a maximum brilliancy. From this it decreases for nearly three months, and after becoming invisible, remains so for five or six months. Its whole period is about 333 days. Are all other stars constant in brightness? The example of _Mira Ceti_ and of other known variables makes this at least doubtful. But the sun itself may vary for all that we know. It is a simple star like the rest.
This question of variability in general is an important one, then. It can only be tested by making accurate catalogues of the relative brilliance of stars at various times, and by comparing these. No such general catalogue existed before HERSCHEL'S time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he made from observation a series of four catalogues, in which were set down the order of sequence of the stars of each constellation.