Unfinished prism, described as 'knife and fork rest' type.

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.