Double prism of glass, after W.J. 's Gravesande

Although the research into optical phenomena has a long history, dating back to the Middle Ages, colours were really understood only after the publication of the colour theory of Isaac Newton. Newton's main conclusion was that white light always contains various colours, and that these colours were not formed - as was supposed at that time - in the material it passed through. To prove it, Newton used two prisms: in the first prism, various colours were generated by the refraction of light, which were bundled into white light again in the second prism, because of an opposite refraction. In this double prism, where the two prisms are pressed together so firmly, that hardly any light is dispersed, Newton's experiment can be demonstrated. The tighter the contact between the two prisms, the lesser dispersion of the light and the more white light will pass through.