Studies on optical experiments to prove Newton’s new theory about light and colours, especially on the so-called Experimentum Crucis, have been accumulated by Richard S. Westfall, J. A. Lohne and many other researchers. Keeping a distance from the issue of ‘crucial experiment’ in the philosophy of science, I would like to trace the history of the role of the Newtonian Experimentum Crucis. To support my opinion, I cite not only Newton’s original writings, but also some reports about replication of Newtonian experiments in the early eighteenth century and some Enlightenment books in the mid-eighteenth century.

Newton proposed Experimentum Crucis, the experiment using two prisms, in his ‘New Theory about Light and Colours’ in 1672. The experiment was a main source of dispute in optics primarily from 1672–1678. After that period, the dispute subsided because of Newton’s silence, and since then Newton did not use the term Experimentum Crucis even in his book Opticks in 1704. Beginning in the 1730s, when Newtonian Enlightenment books, such as Il newtonianismo per le dame ovvero dialoghi sopra la luce e i colori by Francesco Algarotti in 1737, were distributed throughout Europe, the contents of the experiment and the term Experimentum Crucis drew renewed attention. Although the experiment was somewhat problematic in persuading scholars who believed the modification theory of light, Enlightenment books reasserted the experiment and made it part of the scientific mythos.