Brighter than how many suns? Sir Isaac Newton’s burning mirror

There are a number of references in the scientific literature to a burning mirror designed by Sir Isaac Newton (1). Together, they record that it was made from seven separate concave glasses, each about a foot in diameter, that Newton demonstrated its effects at several meetings of the Royal Society and that he presented it to the Society. Nonetheless, neither the earliest published list of instruments possessed by the Royal Society nor the most recent one mentions the burning mirror; the latest compiler does not even include it amongst those items, once owned, now lost. No reference to the instrument apparently survives in the Society’s main records. It is not listed by the author of the recent compendium on Newton’s life and work (2). There is, however, some contemporary information still extant (Appendix 1). Notes of the principles of its design and some of its effects are to be found in the Society’s Journal Book for 1704; some of the dimensions and the arrangement of the mirrors are given in a Lexicon published by John Harris which he donated to the Royal Society at the same meeting, 12 July 1704, at which Newton gave the Society the speculum. The last reference in the Journal Book is dated 15 November that year, when Mr Halley, the then secretary to the Society, was desired to draw up an account of the speculum and its effects (3). No such account appears to have been presented to the Royal Society. There is no reference in Newton’s published papers and letters of his chasing Halley to complete the task, nor is there any mention of it in the general references to Halley. The latter was, of course, quite accustomed to performing odd jobs for Newton; that same year he was to help the Opticks through the press. The only other contemporary reference to the burning mirror, though only hearsay evidence since Flamsteed was not present at the meeting, is in a letter the latter wrote to James Pound; this confirms that there were seven mirrors and that the aperture of each was near a foot in diameter (4). Because John Harris gave his Dictionary to the Royal Society in Newton’s presence, it is reasonable to assume that his description is accurate. As Newton would hardly have left an inaccurate one unchallenged, then, belatedly, the account desired of Mr Halley can be presented. In some respects, the delay is advantageous, since the subject of radiant heat and its effects, although already by Newton’s period an ancient one, is today rather better understood. On