715 



■SL- 



In the application we are going to make of equation (2) n is 

 supposed small, so that we need not integrate over the path of the 

 raj. We suppose in the sun a radially rising, selectively absorb- 

 ing gas mass, in which a velocity gradient exists i)erpendicular to 

 the radius. Even without the density gradients, which are necessary 

 in the theory of Julius, there must be here a deflection of the light 

 waves, especially foi' the wavelengths in the neighi)Ourhood of the 

 absorption lines. 



If we try to work out (luantilatively the idea, on whicii rests 

 equation (2) we directly meet with the diHiculty, that the necessary 

 data are failing. Still we may derive a conclusion from (2), be it 

 with little evidence, viz. that also willi extremely small density of the 

 considered vapour there may exist an observable influence of the 

 Fresnel coefficient on the light waves. 



Let the radially ascending gas mass be found in the centre of 



the visible solar disc and su|)pose that an objective of e. g. 30 cm. 



diameter be used for observation. The light cone proceeding from 



the considered point of the sun has then (the distance of the eaith 



30 



to the sun being 1,5.10''' cm.), a value of = 2.10^"' in 



" ' 1,5 X 1013 



radials. A ray deviating with half this amount from the line that 

 connects the centre of the sun with the objective does not fall in 

 the telescope. A ray to the rim of the objective however needs a 

 deviation of the whole amount to fall beside the telescope. 



For / we may take the depth of the "reversing layer", viz. a 

 number of the order of 1000 k.m. 

 d[i 

 As to — , according to the above mentioned observations of Wood, 

 di. 



this is in the neighbourhood of the sodium line and at 644" C. of 

 the order 10°. The density of sodiuiu vapour is at 644° C. of the order 

 10^5 ^iiiis follows from a calculation, which Mr. C. M. Hoogenhoom, 



46* 



