In 1672 the Royal Society published `A letter of Mr Isaac Newton, Professor of the Mathematics in the University of Cambridge, containing his New Theory about Light and Colours', in which Newton announced that he had performed an `experimentum crucis'. He described the use of a particular combination of two prisms that resulted in the production of the now-familiar oblong spectrum of light. This work of Newton is widely quoted as being the first of its type. However, for many years it has been clear that there were others who might have a right to claim that they had already produced similar work. In spite of this, many scientists and historians continue to quote from Newton's 1672 paper in the preambles to their various works on light and optics. This article is an attempt to bring earlier work to the attention of a wider audience.