Photography, Sir Walter Hartley, and the Discovery of Atmospheric Ozone

Philosopher of science Norwood Russell Hanson (1924-1967) stated the obvious when he wrote “...discovery is what science is all about. Discovery is the “cutting edge” of scientific knowledge.” There is perhaps no tool that has aided in the discovery of more phenomena and that has advanced science across more disciplines than photography. Among the earliest applications of photography to science was in the nascent field of spectroscopy, a branch of science that centers on the analysis of the absorption, reflectance, and emission of electromagnetic radiation as a function of wavelength. The power of spectroscopy as a scientific tool rests on its ability to illuminate the properties of light and to expose the composition and structure of matter. In the earliest applications of spectroscopy, the spectra produced by prisms or diffraction gratings were projected onto a screen and then drawn by hand and analyzed. This was a laborious and inefficient process prone to inaccuracies, as noted by British chemist Sir Walter Noel Hartley (1846-1913): “In the year 1872, when I made a series of observations on the spectra of solutions of cobalt salts, the fatigue to the eye and the labor involved in measuring the position of obscure [spectral] bands, led me to the conclusion that the spectroscope would never be made available for many of the purposes to which it might be applied, until it had been combined with a photographic camera.” Indeed, the camera eventually supplanted drawing for the analysis of spectra, and was central to the discovery of atmospheric ozone. Among the properties first discovered about ozone is that it strongly absorbs UV radiation below 300 nm, a fact that was first established in 1879 by French physicist Marie Alfred Cornu (1841-1902), who combined wet-plate photography with spectroscopic analysis in his experiments. But a couple of years would pass before Sir Walter Noel Hartley combined photography and spectroscopy in experiments that led to his discovery that ozone was responsible for the absorption of UV radiation in Earth’s atmosphere. In this talk, I will discuss Hartley’s pioneering use of dry plates in his photographic-spectroscopic experiments, and the construction, application, and science underlying his camera and lighting systems, which were pivotal to his discovery.