Introduction

In the standard historiography of science the eighteenth century is the period in which the emission conception of light was quite generally accepted, certainly after 1740. Euler is usually mentioned as the exception to this rule. Surveys that are more oriented towards Germany add one or another dissident to the list but leave the image unaltered on the whole: The emission tradition ruled the physical optics roost in the eighteenth century. Apparently the picture of the situation in different countries is to a great extent determined by simply declaring the general picture to be valid for every country, without any thorough investigation of the matter. Britain and Ireland are the only countries on which detailed and systematic research has been carried out. G. N. Cantor has provided an exhaustive survey of optical viewpoints in this region. His results do, it is true, lend nuances to the established image, but they introduce no radical change. In Cantor's book only 9 per cent out of a total of sixty-nine optical theorists from the eighteenth century support a medium theory, while the remaining 91 per cent can be located within the emission tradition. In other words, the historical evidence thus far available confirms the strongly dominant position of the emission tradition in the eighteenth century. Nevertheless, it will be argued in this section that a substantially different view of the matter ought to be given for Germany.

Development of the emission tradition

Newton's suggestions

The theoretical tradition in physical optics in which light is regarded as an emission of matter goes back at least as far as Greek Antiquity and has experienced a renaissance in modern times. Newton was a major representative of the emission tradition in seventeenthcentury optics. In the Enlightenment period his ideas dominated this tradition, although he never held the position of an absolute ruler. Despite his unmistakably great importance for the eighteenth-century emission tradition, Newton had a curious status within it. He never unreservedly endorsed the emission hypothesis, in print at any rate. This is connected with his approach to methodology: He attempted to abstain from combining certainties with doubts. Consequently Newton's ideas on the emission of light are mainly encountered in the form of queries forming a supplement to his Opticks. His ideas remained suggestions that were unconnected with one another, provided no solution to some problems, and were occasionally inconsistent. In addition, they changed over time. For this reason we need a detailed study of the different versions of the ‘queries’ in the various editions of the Opticks if we are to obtain a clear and accurate account of Newton's ideas on the nature of light and of the later attempts to systematize his suggestions. The first edition (Opticks, 1704), the first Latin edition (Optice, 1706), and the second English edition of 1717, are the most important for our purposes.

When Leonhard Euler published his treatise ‘Nova theoria lucis et colorum’ (A new theory of light and colours) in 1746, he made a contribution to the medium tradition in physical optics that was without parallel in the eighteenth century. The ‘Nova theoria’ constitutes the most lucid, comprehensive, and systematic medium theory of that century. The significance of Euler's theory can be gauged partly from the fact that it was so widely discussed. No earlier attempts to provide an alternative to the theories developing within the emission tradition had stimulated pens to the same extent as Euler's ‘Nova theoria’. It is remarkable that a relatively short treatise published as part of a collection of articles on a range of subjects created such resonances, and all the more so when we compare this reaction with the limited response to Huygens' Traite - a complete book - and with the way in which Johann II Bernoulli's prize essay was virtually ignored. A partial explanation no doubt lies in the quality of Euler's work, while his authority also made it difficult to ignore his ‘new theory’.

Euler was probably the most important, or at any rate the most fecund, exponent of mathematics and natural philosophy in the Enlightenment. Although his accomplishments in mathematics and mechanics are generally known and acknowledged, Euler's contributions to optics have attracted little scholarly attention up to the present. However, his role in the discovery of achromatic lenses has been the subject of historical study.

In his survey of historical literature on ‘experimental natural philosophy’ in the eighteenth century, J. L. Heilbron rejects attempts to make the rise of ‘Newtonianism’ and its ultimate triumph over ‘Cartesianism’ the guiding historiographical principle. The results reached in the present study on physical optics parallel Heilbron's argument. We have observed that, in the first half of the eighteenth century, the influence of the opposition between ‘Newtonianism’ and ‘Cartesianism’ in the discussion on the nature of light, also perceptible, was certainly not a dominant feature. Furthermore, there is confirmation for Heilbron's remark that the disciplinary borders of eighteenth-century experimental natural philosophy, especially optics, were subject to change and that it is precisely these changes that can provide us with a new historiographical guideline.

If we pursue the latter suggestion further, we meet with a general thesis advanced by T. S. Kuhn in 1975. Among all the available alternatives, this thesis is to my eyes the most suited to making the various developments in the eighteenth century comprehensible and to collecting them within one general viewpoint. However, when I attempted to use Kuhn's point of view for the clarification of eighteenth-century optics, I found that I could not use it as it stood. In this epilogue I shall propose a corrective to Kuhn's ideas, one that will be illustrated by material derived from the previous chapters. Before doing so, I shall present Kuhn's thesis itself, and examine the way in which he and others using it have described the development of optics.

The aim of this work is to make a two-fold contribution to the study of eighteenth-century science. The majority of this book is devoted to a description and analysis of the conceptual development of physical optics in the period, focussing on the origins, contents, and reception of Leonhard Euler's wave theory of light. There will always be a second question in the background of the narrative, which will receive full attention in the last chapter: What does a study of eighteenth-century optics have to teach us about the changing nature of natural philosophy and science in that period?

The title of this study - Optics in the Age of Euler - constitutes a response to the still generally accepted historical image of optics in which the eighteenth century is portrayed as the century of Newton. According to the standard account, ‘Newton's’ particle, or emission, theory of light dominated for more than a century, whereas ‘Huygens’ wave, or medium, theory supposedly did not develop and found few supporters during the same period. This study provides a corrective to this image, with the Swiss mathematician and natural philosopher Leonhard Euler (1707-83) a leading figure in the new historiographical drama. Euler's importance derives from his “Nova theoria lucis et colorum” (A new theory of light and colours), published in 1746. This article was the foremost eighteenth-century contribution to the development of the medium theories of light. Euler's theory of light, rather than Huygens' theory, was the first serious rival to the emission theories.