386 A Short History of Astronomy [Cn. xin. 



in addition spots were seen to have also independent 

 "proper motions." Carrington also established (1858) the 

 scarcity of spots in the immediate neighbourhood of the 

 equator, and confirmed statistically their prevalence in 

 the adjacent regions, and their great scarcity more than 

 about 35 from the equator; and noticed further certain 

 regular changes in the distribution of spots on the sun in 

 the course of the i i-year cycle. 



Wilson's theory (chapter xii., 268) that spots are de- 

 pressions was confirmed by an extensive series of photographs 

 taken at Kew in 1858-72, shewing a large preponderance 

 of cases of the perspective effect noticed by him ; but, on 

 the other hand, Mr. F. Howlett^ who has watched the sun 

 for some 35 years and made several thousand drawings of 

 spots, considers (1894) that his observations are decidedly 

 against Wilson's theory. Other observers are divided in 

 opinion. 



299. Spectrum analysis, which has played such an im- 

 portant part in recent astronomical work, is essentially a 

 method of ascertaining the nature of a body by a process 

 of sifting or analysing into different components the light 

 received from it. 



It was first clearly established by Newton, in 1665-66 

 (chapter ix., 168), that ordinary white light, such as sun- 

 light, is composite, and that by passing a beam of sunlight 

 with proper precautions through a glass prism it can be 

 decomposed into light of different colours ; if the beam so 

 decomposed is received on a screen, it produces a band of 

 colours known as a spectrum, red being at one end and 

 violet at the other. 



Now according to modern theories light consists essen- 

 tially of a series of disturbances or waves transmitted at 

 extremely short but regular intervals from the luminous 

 object to the eye, the medium through which the disturb- 

 ances travel being called ether. The most important 

 characteristic distinguishing different kinds of light is the 

 interval of time or space between one wave and the next, 

 which is generally expressed by means of wave-length, or 

 the distance between any point of one wave and the corre- 

 sponding point of the next. Differences in wave-length 

 shew themselves most readily as differences of colour ; so 



