ESHS/HSS Annual Meeting

The Search for Genuine ‘Newton’s Prisms’: A Comparison with 18th-Century Dutch Newtonian Prisms of High Dispersion

• In Event: Scientific Instruments

English Abstract

Eight prisms said to have been owned by Isaac Newton (the so-called “Newton’s prisms”) survive today. They are located in the UK and Italy: one at the British Museum, two at the Whipple Museum, one at the Wren Library, one at the Royal College of Physicians, and three at the Treviso Museum. The author measured the refractive indices at nine wavelengths for these eight prisms, thereby determining the dispersion characteristics of each prism. Since previous measurements by Ronchi (1957) and Mills (1981) were limited in scope, this study represents the first comprehensive comparative analysis. The results show that the five prisms preserved in the UK are made of flint glass with higher dispersion than the prisms recorded by Newton in the 1660s. Their optical properties closely resemble those of prisms manufactured by Dutch Newtonians such as s’Gravesande and Musschenbroek in the mid-18th century. These findings clarify the issue that the author raised in a 2013 paper: Why did Newton add a lens to improve his two-prism experiment (the so-called experimentum crucis)? And why did 18th-century Newtonian researchers omit this lens in their popular accounts? Flint-glass prisms became widespread in the early 18th century alongside the dissemination of Newton’s optical theory. The author notes that by the mid-18th century, Newtonians were selling prisms with higher dispersion than the ones Newton himself had actually used, as demonstration apparatus for the new optical theory. This practice may explain why “Newton prisms” inconsistent with Newton’s own records are preserved in museums across Europe.

Author

• Yosimi TAKUWA, Institute of Science Tokyo