Newton's theory of colours

The modification theory of colours

Why is one object red and another blue? Aristotle believed colours to be a mixture of light and darkness. In his view an object is white when all the light striking it is reflected, without the addition of any darkness, and an object is black because it reflects none of the light falling upon it. The colours of objects derive from the mingling of light and darkness in varying proportions. Darkness may originate in something opaque or, as in the case of the rainbow, in an opaque medium, such as the clouds. Red, the purest colour, is a mixture of light and a small amount of darkness. As the amount of darkness increases, green is observed and eventually violet, the ‘darkest’ colour. The other colours consist of a combination of red, green, and violet, the three primary hues. It is fundamental to this interpretation that colours are a modification of pure and homogeneous white light, resulting from the addition of darkness.

The modification theory of colours, which - like so many of Aristotle's ideas - seemed to fit so well with direct observation, was generally accepted until the second half of the seventeenth century, although with variations. Some writers assumed the existence of two or of four primary colours; others opted for three, but chose different hues than Aristotle's. For example, Athanasius Kircher selected yellow, red, and blue.

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.