8 ROUGH WAYS MADE SMOOTH. 



corona : ' A few seconds before the sun was all hid, there 

 discovered itself round the moon a luminous ring about a 

 digit ' (a twelfth) ' or perhaps a tenth part of the moon's 

 diameter in breadth. It was of a pale whiteness or rather 

 pearl colour, seeming to me a little tinged with the colours of 

 the Iris, and to be concentric with the moon.' He added that 

 the ring appeared much whiter and brighter near the body 

 of the moon than at a distance from it, and that its exterior 

 boundary was very ill-defined, seeming to be determined 

 only by the extreme rarity of the luminous matter. The 

 French astronomer Louville gave a similar account of the 

 appearance of the ring. He added, however, that ' there 

 were interruptions in its brightness, causing it to resemble 

 the radial glory with which painters encircle the heads of the 

 saints.' The smallness of the corona on this occasion 

 corresponds with the description of the corona seen in July 

 1878 ; and though Lou villa's description of gaps is sugges- 

 tive of a somewhat different aspect, yet, on the whole, the 

 corona seen in 1715 more closely resembles one which 

 would be seen at a time of minimum solar disturbance, if 

 our theory can be trusted, than one which would be seen at 

 a time of maximum disturbance. Wolfs list puts the year 

 1712 as one of minimum disturbance, with one year of doubt 

 either way, and the middle of the year 1817 as the epoch of 

 maximum disturbance, with a similar range of uncertainty. 

 The case, then, is doubtful, but on the whole inclines to 

 being unfavourable. I may remark that because of its un- 

 favourable nature, I departed from the rule I had set 

 myself, of taking only the cases included in my treatise on 

 the sun. For the corona of 1715 is not described in that 

 treatise, as indeed affording no evidence respecting this 

 solar appendage. The evidence given in this case is pro- 

 bably affected in some degree by the unfavourable atmo- 

 spheric conditions under which Halley certainly, and 

 Louville probably, observed the eclipse. In any case the 

 evidence is not strong ; only I would call attention here to 

 the circumstance that if, as we proceed, we should come to 



