MODERN SCIENCE. 205 



twenty-five years' close observations, and explained their 

 motion to be subject to gravitation, and their revolution to 

 take place around each other, and round a common centre 

 (focus). He also showed that their growing more luminous, 

 and then fainter, was due to their coming nearer to us and 

 then receding further off in the line of our vision. This was- 

 the first evident proof of the law of gravitation holding good 

 throughout the universe, and not merely throughout our solar 

 system. Later investigations have revealed systems com- 

 posed of three and even more stars, and that the increase 

 or decrease of brilliancy in variable stars is due in many 

 cases to other causes than mere motion towards or from us. 

 (See Norman Lockyer and Kirchhoff.) Herschel's theory of 

 the constitution of nebulae has been confirmed, for some 

 nebulae are, as he said, masses of luminous vapours out of 

 which stars are formed, and many others are clusters of stars. 

 But his work went further than this ; for whereas the naked 

 eye can detect 4,100 stars only, .Herschel by means of his 

 powerful telescope could in the course of years compute nearly 

 TWENTY-NINE MILLIONS. Recent computations, supported 

 by photography, make this vast number only a fraction of 

 the whole for it is now thought that 1,000 millions could be 

 mapped. Herschel enunciated the hypothesis that our 

 system belongs to the Milky Way, and is moving along with 

 it towards Hercules, or rather towards X of Hercules, being" 

 attracted thereto at the rate of 150 millions of miles a year by 

 a cluster of some 14,000 suns. He explained the probable 

 constitution of the sun, but his view is no longer accepted. 

 He thought the sun to be a dark body around which lies a 

 luminous atmosphere. The reverse seems rather to be the 

 case. The spectroscope has revealed to us the existence of a 

 central hot and luminous body, called photosphere (light-pro- 

 ducing sphere) ; this is surrounded by an atmosphere, about 

 1,000 miles thick, compose \ of metallic gases (iron, lead, 

 copper, zinc, in fact, most of the metals found on the earth); 

 around this lies a 5,000 mile thick chromosphere (a sphere of 

 hydrogen gas), which shoots flames fifty, seventy, eighty 

 thousand miles high and these eruptions ascend at the rate 

 of 20,000 miles a minute. Beyond and around the chromo- 



