ikt. r.] DISSERTATION SECOND. 187 



writtten near thirty years after the invention of the tele- 

 scope, makes no mention of any but such as is composed of 

 a convex objectglass, and a concave eyeglass. The 

 theory of it, indeed, was given by Kepler in his Dioptricks 

 (1611,) when he also pointed out the astronomical tele- 

 scope, or that which is composed of two convex lenses, 

 and inverts the objects. He did not, however, construct 

 a telescope of that kind, which appears to have been first 

 done by Scheiner, who has given an account of it in the 

 Rosa Ursina (in 1650,) quoted by Montucla. * 



After the invention of the telescope, that of the micro- 

 scope was easy ; and it is also to Galileo that we are indebt 

 edfor this instrument, which discovers an immensity on the 

 one side of man, scarcely less wonderful than that which 

 the telescope discovers on the other. The extension and 

 divisibility of matter are thus rendered to the natural phi- 

 losopher almost as unlimited, as the extension and the di- 

 visibility of space are to the geometer. 



The theory of the telescope, now become the main ob- 

 ject in optical science, required that the law of refraction 

 should, if possible, be accurately ascertained. This had 

 not yet been affected, and Kepler, whose Dioptricks was 

 the most perfect treatise on refraction which had yet ap- 

 peared, had been unable to determine the general princi- 

 ple which connects the angles of incidence and refraction. 

 In the case of glass, he had found by experiment, that 

 those angles, when small, are nearly in the ratio of three to 

 two, and on this hypothesis he had found the focus of a 

 double convex lens, when the curvature of both sides is 

 equal, to be the centre of curvature of the side turned to- 

 ward the object, — a proposition which is known to coin- 

 cide with experiment. From the same approximation, he 

 derived other conclusions, which were found useful in 



. ! Vol. TI. p. 234, 2d edit. 



