Acceptance of Augustin Fresnel's wave theory of light posed numerous questions for early nineteenth-century physicists. Among the most pressing was the problem of the properties of the luminiferous ether. Fresnel had shown that light waves were transverse. Therefore, since, among ordinary materials, only solids support transverse vibrations, there existed striking likenesses between highly tangible solids and the highly intangible ether. Accordingly, such men as Augustin-Louis Cauchy, James MacCullagh, Franz Neumann, and George Green constructed various theories of an elastic-solid ether.1 At (...) the same time, however, the disconcerting implausibilities of an all-pervasive solid provoked considerable apprehension in regard to the elastic-solid tradition. Thomas Young found the concept ‘perfectly appalling’ and argued that ‘the hypothesis [that fluids can support transverse vibrations] remains completely open for discussion, notwithstanding the apparent difficulties attending it.’2 John Herschel, probably the most important English advocate of the wave theory, regarded the concept of a solid ether as only a temporary device, useful ‘till the real truth shall be discovered.’3 Consequently, despite the accomplishments which helped to make the elastic-solid theory ‘the most celebrated special form of the wave theory’,4 there were important voices of reservation. (shrink)

One of the most dramatic advances in the physical sciences during the nineteenth century was the emergence of spectroscopy. It rapidly became an invaluable experimental technique for chemists and astronomers, while for physicists it opened a window upon the world of sub-atomic phenomena. Sir John Herschel played an important part, the value of which has sometimes been underestimated, in the early development of spectroscopy. This paper examines his contribution to the subject during the period 1819–61 in the light of his (...) publications and of certain manuscript material preserved in the Royal Society's Library. Herschel corresponded with most of the scientists who did important work in spectroscopy during his lifetime, and he expressed definite opinions on most of the practical and theoretical problems that arose in it; however, the present study cannot pretend to offer a complete discussion of all aspects of the early history of spectroscopy. (shrink)