Constantijn Huygens Uses Spinoza’s Grinding Dish (1687)

In correspondence Wim Klever directs my attention to evidence that the Huygenses used Spinoza’s grinding equipment as late as 10 years after his death. The citation is here, thus translated from the OCCH:

[I] have ground a glass of 42 feet at one side in the dish of Spinoza’s clear and bright in 1 hour, without once taking it from the dish in order to inspect it, so that I had no scratches on that side ” (Oeuvres completes vol. XXII, p. 732, footnote).

If I have the details here correct, it seems either that indeed the Huygenses had purchased Spinoza’s lens grinding equipment at auction in November of 1677 and maintained the use of that equipment, or that Spinoza may have made a grinding-dish for the brothers under their specification before he died. What is revealed is that Spinoza’s skill had been directed toward not only microscope instruments, but also towards telescopes of a rather large magnitude. This lens appears to have a focal length of 42 ft. And secondly of course, here Constantijn jr., a rather experienced lens-grinder himself, seems to have marveled at the confidence in the lumininocity of the lens produced.

(This reported Spinoza lens is much shorter in focal length than three known to have been made in 1686 by Constantijn: w/ diameters 195, 210 and 230 mm, and w/ focal lengths of 122, 170 and 210 ft.; each “made from the same very poor glass – a heterogeneous and discoloured potash-rich, but essentially lead-free `forest glass’.”)