his position was distinctly less advanced than that of Newton. That great philosopher announced the true law governing the relation between the color and the thickness of the film. Herschel did not recognize such a relation. Newton showed exactly how the phenomenon depended upon the obliquity at which it was viewed. Herschel found no place in his theory for this evident variation.
In the series of experiments described in the first paper on this subject, Herschel mistook the locus of a certain set of rings which he was observing. This mistake, though so slight as hardly to be detected without the guidance of the definite knowledge acquired in later times, not only vitiated the conclusion from the experiments, but gave an erroneous direction to the whole investigation. To him these experiments proved that Newton's conception of a periodic phenomenon was untenable. Thus cut loose from all hypothesis, his fertility in ideas and ingenuity in experimentation are as striking as ever. He tried the effect of having a polished
