CHARTING THE UNIVERSE 



delicacy of their mechanism, not in their principles 

 of action. 



The big forty-inch refracting telescope of Yerkes 

 Observatory, for example, is after all only an over- 

 grown field glass. And the biggest reflecting tele- 

 scope is merely a concave mirror, differing in no 

 essential principle of action from the reflector made 

 by Sir Isaac Newton two centuries ago. As to the 

 remaining implements of the astronomer's essential 

 equipment, the spectroscope was first used in star- 

 testing by Huggins as long ago as 1862; and the 

 utility of the photographic plate has been recognized 

 for about the same period. So it is not so much new 

 methods as the better use of old methods that ac- 

 counts for the spectacular progress of present-day 

 astronomy. 



THE LIGHT-GATHERING TELESCOPE 



The essential function performed by the tele- 

 scope is the gathering of light and bringing it to a 

 true focus. Obviously the larger the lens or mirror, 

 the more light it gathers. But the matter of size is 

 not everything, for the largest of all telescopes, Lord 

 Ross' six-foot reflector, erected in Ireland in 1846, is 

 no match for modern instruments; and the forty- 

 nine-inch lens of the Paris refractor, the biggest thing 

 of its type, has been used only for exhibition pur- 

 poses. It is possible with the use of high power ob- 

 jectives to magnify the telescopic image almost in- 

 definitely. The difficulty is that unless the lens or 

 mirror is very perfect, and the atmosphere very clear, 

 the image becomes only a misty blur. It is not 



33 



