Page:Sir William Herschel, his life and works (1881).djvu/151

Rh A reference to his original papers will show how numerous, how varied, and how valuable these are. I cannot forbear quoting here the account of a precaution observed during his examination of the belts on Saturn (1794).

It is the most striking example of how fully 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.

In making these observations, he says:

"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. 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 6*