Blue glass prism.
This prism comes from a 14 drawer cabinet found in the Herschel family home in the 1950s. Collectively the contents of this and a similar cabinet seems to suggest that they were used by successive generations of the family to store specimens, material and apparatus for carrying out experiments.
This prism is likely to have been bought by William Herschel or possibly made by either William or his brother Alexander, while William was carrying out experiments on the nature and temperature of light. Alternatively, it may have belonged to William's son John or any one of John's 12 children all of whom were taught and pursued scientific interests and all of whom lived for at least some portion of their lives in the Herschel family home in Slough where this piece was found.
National Maritime Museum, Greenwich, London, Herschel Collection