Unpublished letters on Campini's and Divini's telescopes

Historical Significance: This paper reports the discovery of letters by and to Lorenzo Magnalotti in Florence dated 1664 in which is described the use of a measure of visual acuity to measure the resolution of telescopes designed by Divini and Campani. Eustachio Divini (1610-1685) was an Italian manufacturer and experimenter of optical instruments for scientific use; Giuseppe Campani (1635-1715) was an Italian optician and astronomer who lived in Rome during the latter half of the 17th century. This may be the first recorded use of the resolution of letters and words to measure visual acuity albeit for measuring the performance of telescopes rather than the eye.The paper reports that Hooke had investigated keeness of vision by measuring minimum separable and the Spanish optician Daca De Valdes Benito in the early 17th C used counting of seeds from increasing distance as his measure but also used resolution of hand written letters. It was not until the 19thC that optotypes in a scale of sizes were used systematically to measure visual acuity, as a consequence of the work of Kuchler (1835), Jaeger (1854) and Snellen (1862). The paper states that Benito was probably the first optician, having written 'The use of eye glasses for all kinds of sight' in 1623. The picture is one of the visual acuity charts reproduced in the paper.