474 LECTURE XL. 



actually invented a telescope, but that Recorde .himself knew something of 

 its construction. Digges also, in a work published in 1571, has a passage 

 of a similar nature, and from Bacon's own words it has been conjectured that 

 an instrument resembling a telescope was even of much higher antiquity. 

 But the first person, who is certainly known to have made a telescope, is 

 Janson, a Dutchman, whose son, by accident, placing a concave and a convex 

 spectacle glass at a little distance from each other, observed the increased 

 apparent magnitude of an object seen through them; the father upon 

 this fixed two such glasses in a tube a few inches long, and sold the 

 instrument in this form. He also made some telescopes of greater powers, 

 and one of his family discovered a satellite of Jupiter with them. Galileo 

 had heard of the instrument, but had not been informed of the particulars of 

 its construction, he reinvented it in I6O9, and the following year redis- 

 covered 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 fur finding the focal 

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

 ing 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 of view, 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 observa- 

 tions were thirty or forty years later. 



The next great step in optics was made by De Dominis, who in 16II 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 refrac- 

 tion, first made by Snellius, who ascertained, about I62I, that the sines of 

 the angles of incidence and refraction are always in the same proportion to 

 each other at the same surface; he died, however, in 16^6, without having 

 made his discovery public. Descartes, is generally supposed to have 



