In her 1885 history of nineteenth-century astronomy, Agnes Mary Clerke enumerated the discoveries that marked the recent progress in that science. She drew her readers' attention to the founding of what she called ‘astronomical or cosmical physics’, a new species of astronomy that was markedly different in goals as well as methods from its older mathematical cousin. ‘It is full of the audacities, the inconsistencies, the imperfections, the possibilities of youth’, she wrote. ‘It promises everything; it has already performed much; it will doubtless perform much more.’ Clerke was not alone in her enthusiasm.

Britons who played a role in the development of this hybrid discipline, either as active contributors or sideline boosters, hailed it as a happy consequence of the growing interdependency among a heterogeneous subset of scientists and instrument makers. For decades – whether probing the physical properties of light, chemically analysing terrestrial materials or perfecting the production of optical glass – these practitioners had pursued their disparate research agendas using a common, and historically rich, line of investigation: the careful scrutiny of dispersed natural and artificial light. Appropriating elements from the methodological legacy of such notable forebears as Isaac Newton and William Herschel, they examined new types of light sources, tinkered with alternative apparatus arrangements and tested the efficacy of a range of viewing and recording aids. It was a powerful and productive process.