THE SPECTROSCOPE. 297 



appearance, and that the guides were for a time too frightened 

 to proceed. 



THE SPECTROSCOPE. 



NEXT we come to one of the most astonishing and beautiful 

 optical instruments ever made by the hand of man. It is called 

 the Spectroscope, because it deals with a certain arrangement 

 of rays which is called a " spectrum." Many years ago 

 Newton discovered the cause of the lovely colours which 

 deck the rainbow, and the fact that, by passing a ray of 

 white light through a prism, it was decomposed into seven 

 colours, which invariably came in the following order Red, 

 Orange, Yellow, Green, Blue, Indigo, and Violet. He also 

 discovered that, by looking at that coloured band through 

 another prism arranged in a different manner, the decomposed 

 rays were again brought together, and white light was the 

 result. 



Newton had thrown the light on the prism through a round 

 hole, but some time afterwards Dr. Wollaston employed a 

 narrow slit for the purpose, and then found that the spectrum 

 was traversed by dark lines which never changed their places. 

 On these lines depend all the discoveries that have been made 

 by the aid of the Spectroscope. The chief of them are designated 

 by the letters of the alphabet. (See page 300.) 



It was soon found out that if burning gases were viewed 

 with the Spectroscope, lines were still seen, but they were 

 bright instead of dark, and that they invariably occupied the 

 place of one or more of the dark lines shown by the spectrum 

 of sunlight. Then it was discovered that these burning gases 

 absorbed or stopped out the light in the solar spectrum, and 

 from that moment the science rapidly advanced. 



At the present day the Spectroscope not only determines the 

 metals which exist in the sun, but also those of the fixed stars. 

 It even analyzes the constitution of double stars, and shows the 

 reason why one star should be red and the other green. 



One of the most astonishing discoveries in astronomy was 

 due to the Spectroscope. 



During the month of May, 1866, one of the stars in the 

 Northern Crown (Corona Borealis] was seen to undergo a rapid 

 change. It was originally one of the tenth magnitude, but in 



