Our eyes are capable of seeing only a narrow spectrum of light.
The dark lines of the spectrum of sunlight, earliest noted by Dr W.
The spark spectrum of gold has been mapped by A.
The question is whether, when the adjustment of focus is correct for the central rays of the spectrum, the error of phase for the most extreme rays (which it is necessary to consider) amounts to a quarter of a wave-length.
The study of the latter he took up as a result of the publication in 1871 of an energy-curve of the spectrum by S.