392 A Short History of Astronomy [Cn. xin. 



a way that its wave-length is apparently decreased by 

 J^-Q part, it may be inferred that the body is approach- 

 ing with the speed just named, or about 186 miles per 

 second, and if the wave-length appears increased by the 

 same amount (the line being displaced towards the red end 

 of the spectrum) the body is receding at the same rate. 



Some of the earliest observations of the prominences by 

 Sir J. N. Lockyer (1868), and of spots and other features 

 of the sun by the same and other observers, shewed dis- 

 placements and distortions of the lines in the spectrum, 

 which were soon seen to be capable of interpretation by 

 this method, and pointed to the existence of violent dis- 

 turbances in the atmosphere of the sun, velocities as 

 great as 300 miles per second being not unknown. The 

 method has received an interesting confirmation from obser- 

 vations of the spectrum of opposite edges of the sun's disc, 

 of which one is approaching and the other receding owing 

 to the rotation of the sun. Professor Duner of Upsala has 

 by this process ascertained (1887-89) the rate of rotation 

 of the surface of the sun beyond the regions where spots 

 exist, and therefore outside the limits of observations such 

 as Carrington's ( 298). 



303. The spectroscope tells us that the atmosphere of 

 the sun contains iron and other metals in the form of 

 vapour ; and the photosphere, which gives the continuous 

 part of the solar spectrum, is certainly hotter. Moreover 

 everything that we know of the way in which heat is com- 

 municated from one part of a body to another shews that 

 the outer regions of the sun, from which heat and light are 

 radiating on a very large scale, must be the coolest parts, 

 and that the temperature in all probability rises very rapidly 

 towards the interior. These facts, coupled with the low 

 density of the sun (about a fourth that of the earth) and 

 the violently disturbed condition of the surface, indicate that 

 the bulk of the interior of the sun is an intensely hot and 

 highly compressed mass of gas. Outside this come in order, 

 their respective boundaries and mutual relations being, how- 

 ever, very uncertain, first the photosphere, generally regarded 

 as a cloud-layer, then the reversing stratum which produces 

 most of the Fraunhofer lines, then the chromosphere and 

 prominences, and finally the corona. Sun-spots, faculae, and 



