Note 28 — Chapter IV · Of the Measure of Secondary Qualities.

The reference to Fraunhofer’s Lines, as a means of determining the place of a colour in the prismatic series, has been objected to, because, as is asserted, the colours which are in the neighbourhood of each line vary with the position of the sun, state of the atmosphere and the like. It is very evident that coloured light refracted by the prism will not give the same spectrum as white light. The spectrum given by white light is of course the one here meant. It is an usual practice of optical experimenters to refer to the colours of such a spectrum, defining them by Fraunhofer’s Lines. I do not know whether it needs explanation that the ‘first spectrum’ in Newton’s rings is a ring of the prismatic colours. I have not had an opportunity of consulting Lambert’s Photometria, sive de mensura et gradibus luminis, colorum, et umbræ, published in 1760, nor Mayer’s Commentatio de Affinitate Colorum, (1758), in which, I believe, he describes a chromatometer. The present work is not intended to be complete as a history; and I hope I have given sufficient historical detail to answer its philosophical purpose.