164 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



It must never be forgotten (but it often is) that in our obser- 

 vations we are dealing with a sphere, and not with a section. So 

 that if we imagine a layer to exist at any height above the sun, 

 its spectral lines will be visible down to the spectrum of the sun 

 itself. Whether, therefore, there is such a layer is very difficult 



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H 



Su n's 

 ec t rum 



FIG. 67. Diagram showing that if the sun's atmosphere consists of layers the 

 lines will still extend down to the solar spectrum. For the layer A is also 

 represented at B and B', so that we shall see (2) in the spectrum and not (1). 

 If thickness of stratum increased the width of the line, we should see (3) 

 if the layer A did not extend to the sun, and (4) if it did. 



of determination if we neglect to notice the various thicknesses 

 of the lines. In fact the thickening towards the sun seems 

 the only criterion ; if we suppose it not to exist, then it is 



Sun's 



Spectrum 



FIG. 68. Spectral phenomena on the assumption of three layers, c, outermost 

 layer, being represented at c', will give us a long line as at c, as it would do 

 if the stratum extended from c to A along the line x Y, and so with B. 



easy to see that there is no spectroscopic method of determining 

 the existence of the layers if we neglect temperature. 



If we t^ke temperature into consideration, then, as the lines 

 will be less bright as the distance from the sun is increased, 



