106 MORRIS LOEB 



in recent days that all these imperfections have been cor- 

 rected, and we now have glass lenses of forty inches' diameter, 

 as in the great Chicago telescope. But Newton invented a 

 telescope which was quite independent of glasses, using the 

 principle which you can discover for yourselves by looking 

 into the bowl of an ordinary tablespoon. The reflecting 

 telescope has been much used, and the highest point reached 

 in its construction was Lord Rosse's instrument, which was 

 fifty feet long, six feet in diameter, and possessed a mirror 

 weighing six tons. 



Yet another practical device comes even more closely 

 home to us. Navigators are indebted to Newton for the in- 

 strument next in importance to the compass, the sextant. 



The chromatic errors of lenses led Newton to study the 

 nature of light so profoundly that he not only avoided the 

 errors of his predecessors but also profited by them. Allowing 

 a beam of sunlight to pass through the round hole of a shutter 

 and fall upon a prism, he found that it was broken up, and 

 that in place of a single white dot it became a streak of various 

 colors. From this he inferred that white light is not simple but 

 consists of various rays that can be separated by a prism. He 

 attempted to recompose the light out of the colors into which 

 he had dissociated it, and found here again that he was suc- 

 cessful. From this discovery of Newton's there have been 

 distinct advances in later times. It is found better to re- 

 place the round hole in the shutter by a slit. We then find 

 that white light is decomposed to give a band of purer 

 colors than that obtained through a round opening. It is 

 called a spectrum, and is well known to most of you. This 

 band can be shown to be made of little images of the slit 

 placed side by side. Afterwards Fraunhofer showed that if 

 the slit was made narrow enough, a large number of fine lines 

 appeared across the spectrum. And later still, Bunsen and 



