The Evolution of the Sciences 



later chapter, how the observation of total 

 eclipses of the sun has enabled the solar atmos- 

 phere to be observed with the naked eye; but 

 this exceptional method is not the only one 

 enabling us to study it. In 1868 M. Janssen 

 and Sir Norman Lockyer discovered simultane- 

 ously the method which has rendered possible 

 the daily observation of the chromosphere. It 

 consists in forming an enlarged image of the 

 solar disc, against the edge of which is turned 

 the slit of the spectroscope, that is to say, the 

 narrow rectilinear opening through which the 

 light to be analysed enters the instrument. It is 

 a delicate experiment, because the zone to be 

 observed forms a very thin layer around the 

 photosphere. As long as the disc itself is ex- 

 amined the usual bright spectrum appears 

 crossed by dark lines, but the moment the 

 photosphere is left behind the image changes 

 entirely; the field of the spectroscope becomes 

 darkened, with the exception of a few bright 

 lines characteristic of a gaseous mass at a high 

 temperature. The existence of the chromo- 

 sphere is thus demonstrated by a direct and 



decisive experiment; it only remains to deter- 



178 



