Kepler had heard of Hariot’s acquirements in Natural Philosophy from his friend John Eriksen. Would be glad to know Hariot’s views as to the origin and essential differences of colours; also on the question of refraction of rays of light; and the causes of the Rainbow; and of haloes round the sun. _Letter_ 223, _dated London,_ 11 _December, 1606,from_ _Thomas Hariot to John Kepler,_ Had received with pleasure Kepler’s letter; but should not be able to answer it at length, being in indifferent health, so that it was not easy to write or even carefully to reflect. Sends a table of the results of experiments on equal bulks of various liquids and transparent solids (thirteen in number, including spring, rain, and salt water; Spanish and Rhenish wine; vinegar; spirits of wine; oils and glass). The angle of incidence is 30° in each case; also the specific gravity of each substance is given. Then he discusses the reason why refraction takes place. Promises to write on the Rainbow; but will merely say at present that it is to be explained by the reflection on the concave superficies and the refraction at the convex superficies of each separate drop. _Letter_ 224 _is from John Kepler to Thomas Hariot, dated at Prague,_ 11 _August,_ 1607. Thanks Hariot for his table, which supplies matter for serious consideration. Asks questions as to how he defines the angles of incidence and refraction; and goes on to discuss the reasons of refraction. Agrees with Hariot as to his views about the Rainbow; but will be very glad to receive his treatises on Colours and the Rainbow. _Letter_ 225 _is from Thomas Hariot to John Kepler, dated at Syon,_ _near London,_ 13 _July_ (o.s.), 1608.