The Sun 



regarding their actual form ; the spots are cavities 

 with sloping sides pierced through the external 

 luminous zone and opening upon a dark interior 

 region. As a matter of fact, when a slowly 

 changing spot of regular circular form is fol- 

 lowed in its motion across the disc it will be 

 seen, as it approaches the edge, to assume an 

 oval form, then to narrow until it becomes 

 linear. This series of appearances agrees 

 entirely with the projection effects of a hole 

 observed first perpendicularly and then under 

 increasing degrees of obliquity; this observation, 

 which has been checked repeatedly by various 

 astronomeis, compels us to discard every ex- 

 planation but Wilson's. 



Hitherto we have examined as a whole the 

 light emitted by different points of the sun ; we 

 shall now analyse the complex radiations which 

 emanate from it. As is well known, the use of 

 the prism enables this classification to be made. 

 Kirchhoff and Bunsen, the true creators of spec- 

 trum-analysis, invented an instrument, the spec- 

 troscope, based on this property of the prism 



whose employment enables us to characterise 



169 



