Optics in the Age of Euler

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.