In an article published recently in this journal, one of us reconstructed how in 1899 J. J. Thomson, after having measured the mass-to-charge ratio of the corpuscle , achieved a measurement of its charge and consequently an estimate of its mass, obtaining in this manner ‘direct proof of the existence of particles smaller than the hydrogen atom’. In this paper, starting with an analysis of Zeeman's first measurements on the widening of spectral lines in a magnetic field, we show that (...) the existence of masses smaller than the atom was already a matter of discussion in early 1897, and that a qualitative estimate of the mass of the electron was made even before Thomson's measurements in 1899. In this process an important role was played by a research programme on the structure of matter carried out by Stoney in the latter part of the nineteenth century. This had in many aspects anticipated Zeeman's experimental observations, and its conclusions were in part formalized in the so-called Larmor theorem. (shrink)

On 30 April, 1897, J. J. Thomson announced the results of his previous four months' experiments on cathode rays. The rays, he suggested, were negatively charged subatomic particles. He called the particles ‘corpuscles’. They have since been re-named ‘electrons’ and Thomson has been hailed as their ‘discoverer’. Contrary to the accounts of most later writers, I show that this discovery was not the outcome of a concern with the nature of cathode rays which had occupied Thomson since 1881 and had (...) shaped the course of his experiments during the period 1881–1897. An examination of his work shows that he paid scant attention to cathode rays until late 1896. (shrink)

A hundred years ago the science of spectroscopy, though not yet christened, may be said to have attained its majority and to be just entering on its period of full adult development. It was born, of course, with Newton's explanation of the formation of the spectrum, and for many years thereafter little of importance was added to what he had discovered. It was not, in fact, until the nineteenth century that anything of outstanding importance occurred, and then, in 1802, Wollaston (...) substituted a slit for the round hole through which Newton's sunlight passed into his prism, and thereby not only saw for the first time the dark lines in the solar spectrum but also took the first step towards the perfection of the spectroscope on which all later progress depended. The next step was taken by Fraunhofer who, in 1814, examined the spectrum through a telescope instead of letting it fall on a screen. The last essential improvement—the introduction of the collimator to make the light from the slit parallel before it entered the prism—was introduced in 1839 independently by Simms and Swan, so that before our period begins, the complete spectroscope existed, though it was not to be converted into a spectrograph, for photographing spectra, until much later. (shrink)