The departure of Eriksen and other matters do not allow leisure to write at length. The turpentine (oleum terebinth inum) was not the same as that experimented on by Kepler but a purer and lighter article (Sp. grav. ’87). The angle of incidence is understood as defined by Alhazen and Vitellio [first published 1572]. Points out some errors in Vitellio’s second table of refractions. As to the causes of refraction, Hariot believes in the theory of the vacuum; ‘where we still stick in the mud’. Hopes God (Deum optimum maximum) will soon put an end to this. Wishes for Kepler’s meteorological records for the last two years, and will send his own notes in return. Gilbert, author of a work on the magnet, had recently died, leaving in his brother’s hands a book entitled ‘De Globo et Mundo nostro sub lunari Philosophia nova contra Peripateticos, lib. 5." [A treatise, in five books, on Natural Philosophy, in answer to the Peripatetics.] The book is likely to be published before the end of the year. Hariot had read some chapters; and saw that Gilbert defends the doctrine of a vacuum. Not to leave a vacuum on this page (says Hariot), it is remarkable that though gold is both heavy and opaque, when beaten out into gold-leaf the light of a candle can be seen through it, though it appears of a green colour. _Letter 226, from John Kepler to Thomas Hariot, it dated from_ _Prague, September,_ 1609. Excuses himself for not having replied sooner; having been very busy; but would not lose the present opportunity of writing. Discusses the questions of refraction and the vacuum. Commentaries on Mars entitled ‘Astronomia Nova [Greek Text] or Physica Cælestis,’ have been published at Frankfort; has not a copy by him. Regrets to hear of the death of Gilbert. Hopes his work on Magnetism will also be published; and that Erikson will bring a copy with him. Promises to send a copy of his own meteorological observations; and hopes to receive Hariot’s. These studies in optics and this correspondence with the learned Kepler indicate Hariot’s great advancement in natural philosophy as early as 1606 to 1609 and give an earnest of his inventive genius and scientific enterprise with his telescope in the astronomical discoveries which immediately followed in 1609 to 1613. Before awarding all the prizes for discoveries and inventions in mathematics, philosophy and natural science to claimants throughout the wide Republic of Letters, let modest Hariot be heard and examined. Let his papers and all his credentials be laid out before the high court of science, not in the light of today, but contemporaneously with those of Tycho, Kepler, Galileo, Snell, Vieta and Descartes. Hariot himself has claimed nothing, but Justice and Historical Truth are bound to assign him a niche appropriate to his merits.