376 LECTURE XL. 



family discovered a satellite of Jupiter with them.* Galileo t had heard 

 of the instrument, but had not been informed of the particulars of its con- 

 struction ; he reinvented it in 1609, and the following year;}; rediscovered 

 also the satellite which Janson had seen a little before. 



It was, however, Kepler that first reduced the theory of the telescope 

 to its true principles ; he laid down the common rules for finding the focal 

 lengths of simple lenses of glass ; he showed how to determine the magni- 

 fying power of the telescope, and pointed out the construction of the simple 

 astronomical telescope, which is more convenient for accurate observations 

 than the Galilean telescope, since the micrometer may be more easily applied 

 to it ; a third glass, for recovering the erect position of the object, was after- 

 wards added by Scheiner, and a fourth, for increasing the field ofview, by 

 Rheita. Kepler made also some good experiments on the nature of coloured 

 bodies, and showed the inverted situation of the image formed on the retina 

 of the eye. Maurolycus || of Messina had demonstrated, in 1575, that the 

 pencils of light are brought to focal points on the retina ; Kepler's obser- 

 vations were thirty or forty years later. 



The next great step in optics was made by De Dominis,1F who in 1611 

 first explained the cause of the interior or primary rainbow, and this was 

 soon followed by a still more important discovery respecting the nature of 

 refraction, first made by Snellius, who ascertained, about 1621, that the 

 sines of the angles of incidence and refraction are always in the same pro- 

 portion to each other at the same surface ; he died, however, in 1626, 

 without having made his discovery public. Descartes is generally supposed 

 to have seen Snellius's papers, although he published the law of refraction** 

 without acknowledging to whom he was indebted for it. Descartes also 

 explained the formation of the secondary rainbow, ft and truly determined 

 the angular magnitude of both the bows from mathematical principles ; he 

 did not, however, give a sufficient reason for the production of colours in 

 either case. Descartes imagined light to consist in motion, or rather pres- 

 sure, transmitted instantaneously through a medium infinitely elastic, and 

 colours he attributed to a rotatory motion of the particles of this medium.^ 

 He supposed that light passed more rapidly through a denser medium than 

 through a rarer; other philosophers about the same time maintained a 

 contrary opinion, without deciding with respect to any general theory of 

 light : thus Fermat and Leibnitz deduced, on this supposition, the path of 

 refracted light from the natural tendency of every body to attain its end 



* Borellus, De Vero Telescopii Inventore, 4to, Hagse, 1655, p. 40. ThatBo- 

 rellus had no just grounds for this statement is shown by Moll, Journal of the Roy. 

 Inst. Nos. 2 and 3 ; and Drinkwater, Ph. Mag. 1832, i. 14. The credit of the dis- 

 covery of Jupiter's satellites is certainly due to Galileo. 



t Opere, ii. 4. 



Ibid. p. 17, and Nuncius Sidereus, Venet. 1610. 



Dioptrice, 4to, Augsb. 1611. 



II Theoremata deLumine, 4to, Lugd. 1613. 



1[ De Radiis visis in Iride, 4to, Venet. 1611. But see Descartes, Meteorum, cap. 

 viii. p. 196. 



* Specim. Dioptrices, chap. ii. 7. See Huygens, Dioptrica, p. 2. 



ft Spec. Meteorum, chap. viii. ++ De Lumine, chap. i. 



