CHAPTER VII. 



The Effect of a Refraction in the Solar Atmosphere. 



A The Longitude Motions of the Spots at different Distances from 

 the Central Meridian. 



The existence of a refraction in the solar atmosphere causing the velocity of 

 the spots apparently to decrease with increasing distance from the centre, has been 

 previously pointed out. A correction for this effect of x'efraction is applied 

 to the coordinates of sun-spots published in Publication en des Astro- 

 physikalischeu Observatoriums zu Potsdam. Any such correction, which 

 must in fact be rather approximately determined, has, on tlie contrary, not been 

 applied to the measurements of the photographs taken at Greenwich, which, as 

 before stated, are the basis of the present investigation. 



Since the size of the refraction is dependent on the distance of the spot from 

 the centre of the sun, it may evidently be determined by an examination of the 

 motion of the spots at different distances from this centre. As I did not intend 

 at first to investigate this problem, I have not completely accomplished such a 

 division of the material of observation. Some remarks on this question may, 

 however, be of some interest. 



We have found in the preceding chapter that the two halves of the sun in 

 some respects presented certain dissimilarities. This induced me to examine whether 

 the observations of the spots on both sides of the central meridian should give 

 different values of the angular velocity. The differences between the mean values 

 of the longitude motions on the western and eastern halves amount, for the two 

 hemispheres, during the first peiiod to -|- 0.'20l and f 0.'l95, during tlie second to 

 -[- 0.'244 and -\- 0.'240. The mean errors only amounting to O.'OBO, this difference 

 cannot be accidental. 



On account of this difference I made a further division of the material accor- 

 ding to the distance from the central meridian. In tliis division I have taken 

 both periods as well as both hemispheres together. The results of the computations 

 are brought together in table XXXIX. As seen from this table, there exists an evident 



Lands Universitets Årsskrift. N. F. Afd. 2. Bd lü. 12 



