- 1 - A Notion or a Measure: The Quantification of Light to 1939 by Sean François Johnston Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds Department of Philosophy Division of History and Philosophy of Science November, 1994 The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the work of others. - 2 - Abstract This study, presenting a history of the measurement of light intensity from its first hesitant emergence to its gradual definition as a scientific subject, explores two major themes. The first concerns the adoption by the evolving physics and engineering communities of quantitative measures of light intensity around the turn of the twentieth century. The mathematisation of light measurement was a contentious process that hinged on finding an acceptable relationship between the mutable response of the human eye and the more easily stabilised, but less encompassing, techniques of physical measurement. A second theme is the exploration of light measurement as an example of ‘peripheral science’. Among the characteristics of such a science, I identify the lack of a coherent research tradition and the persistent partitioning of the subject between disparate groups of practitioners. Light measurement straddled the conventional categories of ‘science’ and ‘technology’, and was influenced by such distinct factors as utilitarian requirements, technological innovation, human perception and bureaucratisation. Peripheral fields such as this, which may be typical of much of modern science and technology, have hitherto received little attention from historians. These themes are pursued with reference to the social and technological factors which were combined inextricably in the development of the subject. The intensity of light gained only sporadic attention until the late nineteenth century. Measured for the utilitarian needs of the gas lighting industry from the second half of the century, light intensity was appropriated by members of the nascent electric lighting industry, too, in their search for a standard of illumination. By the turn of the century the ‘illuminating engineering movement’ was becoming an organised, if eclectic, community which promoted research into and standards for the measurement of light intensity. The twentieth-century development of the subject was moulded by organisation and institutionalisation. Between 1900 and 1920, the new national and industrial laboratories in Britain, America and Germany were crucial in stabilising the - 3 - subject. In the inter-war period, committees and international commissions sought to standardise light measurement and to promote research. Such government- and industry-supported delegations, rather than academic institutions, were primarily responsible for the ‘construction’ of the subject. Practitioners increasingly came to interpret the three topics of photometry (visible light measurement), colorimetry (the measurement of colour) and radiometry (the measurement of invisible radiations) as aspects of a broader study, and enthusiastically applied them to industrial and scientific problems. From the 1920s, the long-established visual methods of observation were increasingly replaced by physical means of light measurement, a process initially contingent on scientific fashion more than demonstrated superiority. New photoelectric techniques for measuring light intensity engendered new commercial instruments, a trend which accelerated in the following decade when photometric measurement was applied with limited success to a range of industrial problems. Seeds sowed in the 1920s – namely commercialisation and industrial application, the transition from visual to ‘physical’ methods, and the search for fundamental limitations in light measurement – gave the subject substantially the form it was to retain over the next half-century. - 4 - Contents Abstract ..............................................................................................................ii Contents .............................................................................................................iv Figures ...........................................................................................................viii Tables ..............................................................................................................x Abbreviations............................................................................................................xi Acknowledgements..................................................................................................xv Chapter 1 Introduction ..........................................................................................1 Organisation of this thesis ..................................................................10 Scope.........................................................................................10 Sources......................................................................................12 Terms ........................................................................................15 Chapter 2 The Prehistory of Light Measurement ...............................................17 Beginnings..........................................................................................17 Light as a law-abiding quantity..........................................................24 Photography: juggling variables...............................................26 Astronomy: isolated forays.......................................................28 Techniques of visual photometry .......................................................29 Studies of radiant heat ........................................................................31 Colour measurement...........................................................................33 Chapter 3 Towards Quantitative Measurement ..................................................36 Recurring themes................................................................................37 Changes of approach after 1860.........................................................40 Astrophysics and the scientific measurement of light ..............40 Spectroscopy.............................................................................47 Standards, gas and electrotechnical photometry.......................48 The nineteenth century photometer....................................................57 Problems of visual intensity measurement.........................................62 Quantifying light: n-rays vs. blackbody radiation..............................68 Chapter 4 The Organisation of Light Measurement ...........................................75 Amateurs and independent research...................................................76 Illuminating engineering in Britain and America...............................79 - 5 - Optical societies .................................................................................93 Chapter 5 Photometry Institutionalised ..............................................................97 The drive of utilitarian need ...............................................................97 The Physikalisch-Technische Reichsanstalt.....................................100 The National Physical Laboratory....................................................104 The National Bureau of Standards ...................................................108 Colour at the national laboratories ...................................................112 Career paths......................................................................................116 Comparison of the national laboratories ..........................................120 Industrial laboratories.......................................................................123 Photometry and World War I ...........................................................127 Consolidation of practitioners ..........................................................129 Chapter 6 Technology in Transition ................................................................131 Perceptions of physical photometry .................................................132 The development of visual photometry............................................137 The replacement of visual by photographic methods.......................141 Physical photometry for astronomers...............................................144 An awkward hybrid: photographic recording and visual analysis.....................................................................144 A half-way house: photographic recording and photo- electric analysis ........................................................147 A ‘more troublesome’ method: direct photoelectric photometry ...............................................................149 The general adoption of photoelectric photometry ..........................154 Recalcitrant problems.......................................................................162 Linearity........................................................................................ 162 The spectre of heterochromatic photometry...........................164 Chapter 7 Light and Colour Measurement by Delegation................................167 The Commission Internationale de Photométrie..............................169 The Commission Internationale de l’Éclairage................................171 Legislative connections ....................................................................177