In his recollections Newton describes how he made his greatest discovery:
In August 1665, Sir I., who was not then not 24 brought at Sturbridge fair a Prism to try some experiments upon Descartes's book on colors & when he came home he made a hole in his shutter & darkened the room & put his Prism between that & the wall... etc.
My question: who and for what reason was making prisms and selling them in the market? There must have been some demand. Who was buying them them and for what purpose?
Edit. My earlier naive conjecture was that there were some scientific equipment makers catering to Cambridge scientists:-) But this is refuted by two facts: a) First of all this did not happen in Cambridge. Newton was at that time on quarantine in his mother's estate, far from Cambridge. And b) scientists in Cambridge were not interested at that time in scientific experiments: they studied mostly theology and Aristotle's writings.
Westfall in his most comprehensive book about Newton, "Never at rest", writes: "Newton's recollection of Sturbridge Fair may have been mistaken, there was also annual Midsummer Fair which managed to escape both plagues. If he purchased the prism there in 1665, he could have taken it home with him and performed there basic, though perhaps crude, experiments connected with his initial insight".
He also mentions that "prisms seem an unlikely item of commerce for a small market town" but does not elaborate on this.