ASTRONOMY, ETC., OF SEVENTEENTH CENT. 217 



mirror for the small concave one. Newton devoted much attention 

 to the grinding of lenses and mirrors, and, acquainted with Gregory's 

 proposed construction of a reflecting telescope, he in 1668 conceived 



ir- 



FlG. 102. 



that it would be an improvement not to pierce the mirror, but to re- 

 flect the rays coming from it to the side of the tube by means of a 

 plain oval mirror. This construction is shown in Fig. 103. Newton 

 constructed with his own hand a very small telescope in which this 

 arrangement was adopted, and the instrument, which was only 6 inches 

 in length, possesses the special interest of being the first reflecting 

 telescope ever executed. The success of this attempt inspired Newton 



FIG. 103. 



to prepare another reflecting telescope of larger size, and some time 

 afterwards this instrument became the property of the Royal Society. 

 It is still preserved in the Society's rooms, and its pedestal bears the 

 inscription, " Invented by Sir Isaac Newton, and made with his own 

 hands, 1671." Newton had afterwards the satisfaction of seeing his 

 invention become an instrument of scientific utility. 



We shall now proceed to describe Newton's discoveries regarding 

 Light discoveries which are almost as remarkable for scientific saga- 

 city as those which relate to universal gravitation. Newton informs 

 us, that in 1666 he "procured a triangular glass prism, to try there- 

 with the celebrated phenomena of colours," intending probably to 

 repeat the experiment which Descartes had described (see page 160), 

 merely as proving that the rainbow is produced by refraction. Newton's 

 experiment was thus arranged : he made a -small round hole in the 

 window-shutter of a darkened room, so as to admit a ray of sunlight, 

 A c, Fig. 104 ; in the path of this ray he placed the prism c, and this 

 so refracted the ray, that on a white screen placed on the opposite wall 

 appeared a coloured image, about five times as long as it was broad. 

 These colours of this image or spectrum were always arranged in the 



