THE SUN'S CORONA AND HIS SPOTS. n 



as were seen in 1870 and 1871. Now if this peculiarity 

 should really be attributed to the relation existing between 

 the corona and the sun-spots, we should infer that in 1766 

 the corona would have been still more markedly uniform in 

 shape. The existence of four well marked expansions on 

 that occasion forces us to assume that either the relation 

 referred to has no real existence, or else that the corona 

 may change from week to week as the condition of the sun's 

 surface changes, and that in February, 1766, the sun was 

 temporarily disturbed, though the year, as a whole, was one 

 of minimum disturbance. But as the epoch of actual 

 minimum was the middle of 1766, February 1766 should 

 have been a time of very slight disturbance. I do not 

 know of any observations of the sun recorded for the 

 month of February, 1766. On the whole, the eclipse of 

 1766 must be regarded as throwing grave doubt on the 

 relation assumed by our theory as existing between the 

 corona and the sun-spots ; and as tending to suggest that 

 some wider law must be in question than the one we have 

 been considering if any association really exists. 



The account given by Don Antonio d'Ulloa of the 

 appearance presented by the corona during the total eclipse 

 of 1778, is rendered doubtful by his reference to an apparent 

 rotatory motion of the normal rays. He says that about 

 five or six seconds after totality had begun, a brilliant lumin- 

 ous ring was seen around the dark body of the moon. The 

 ring became brighter as the middle of totality approached. 

 * About the middle of the eclipse, the breadth of the ring 

 was equal to about a sixth of the moon's diameter. There 

 seemed to issue from it a great number of rays of unequal 

 length, which could be discerned to a distance equal to the 

 moon's diameter.' Then comes the part of d'Ulloa's de- 

 scription which seems difficult to accept. He says that the 

 corona ' seemed to be endued with a rapid rotatory motion, 

 which caused it to resemble a firework turning round its 

 centre.' The colour of the light, he proceeds, 'was not 

 uniform throughout the whole breadth of the ring. Towards 



