wave-length, he in this memoir supposed the frequency greatest for red light, and least for violet; but a few years later[1] he adopted the opposite opinion.
The chief novelty of Euler's writings on light is his explanation of the manner in which material bodies appear coloured when viewed by white light; and, in particular, of the way in which the colours of thin plates are produced. He denied that such colours are due to a more copious reflexion of light of certain particular periods, and supposed that they represent vibrations generated within the body itself under the stimulus of the incident light. A coloured surface, according to this hypothesis, contains large numbers of elastic molecules, which, when agitated, emit light of period depending only on their own structure. The colours of thin plates Euler explained in the same way, the elastic response and free period of the plate at any place would, he conceived, depend on its thickness at that place; and in this way the dependence of the colour on the thickness was accounted for, the phenomena as a whole being analogous to well-known effects observed in experiments on sound.
An attempt to improve the corpuscular theory in another direction was made in 1752 by the Marquis de Courtivron,[2] and independently in the following year by T. Melvill.[3] These writers suggested, as an explanation of the different refrangibility of different colours, that "the differently colour'd rays are projected with different velocities from the luminous body: the red with the greatest, violet with the least, and the intermediate colours with intermediate degrees of velocity." On this supposition, as its authors pointed out, the amount of aberration would be different for every different colour; and the satellites of Jupiter would change colour, from white through green to violet, through an interval of more than half a minute before their immersion into the planet's shadow; while at emersion the contrary succession of colours should be observed,
