360 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



X. An Ansiuer to the Objections stated by M. de la Lande, in the Memoirs of the 

 French Academy for J 776. against the Solar Spots being Excavations in the 

 Luminous Matter of the Sun, with a short Examination of the Views enter- 

 tained by him on that Subject. By Alex. Wilson, M. D. Prof, of Practical 

 Astron. Glasgow, p. 144. 



In the first part of his paper, published in the Philos. Trans, for 1774, Dr. W. 

 explained how, from the lucky accident of seeing the great solar spot of Nov. 

 176Q, in a certain critical situation on the disk, its real nature was obtruded on 

 his thoughts by a train of appearances the most obvious and unequivocal. It 

 may there also be seen how, from phenomena perfectly similar in spots of the 

 usual size, he was led to a general conclusion, and to believe that all spots, small 

 as well as great, which consist of a dark nucleus, and surrounding umbra, are 

 excavations in the luminous matter of the sun. Having however lately seen, in 

 the Memoirs of the French Academy for 1776, published in 177Q, that his 

 paper on the Solar Spots has come under the notice of a member of that illustri- 

 ous body, M. de la Lande, Dr. W. here intends very freely to offer what argu- 

 ments occur to him in favour of the solar spots being such as he had described. 

 First of all, it has been urged, as an objection of great weight, that the absence 

 of the umbra on one side, when spots are near the limb, as so fully explained in 

 Dr. W.'s paper, is not constant. As to the fact, it may be there seen, that he 

 was sufficiently aware of it, having stated 3 cases from his own observations, when 

 he did not perceive this change to take place. The Rev. Mr. VVollaston is the 

 only person who, in the Philos. Trans, has bestowed any remarks on his publica- 

 tion ; and though he with great candour acknowledges, that generally the umbra 

 changes in the manner Dr. W. had determined, yet he expresses a difficulty as to 

 the Dr.'s conclusions, on account of this circumstance not obtaining universally. 



Under similar expressions, says Dr. W. M. de la Lande produces from his own 

 observations, which appear to have been long continued, only 3 cases of the same 

 kind, and from the ancient observations of Mess. Cassini and De La Hire, 4 

 more ; which Dr. W. objects against as not sufficiently described or insisted on 

 by these astronomers. But even admitting this anomaly to be much more fre- 

 quent than can be contended for, still such cases, Dr. W. says, can only be 

 brought as so many exceptions from a certain general law, or uniformity of ap- 

 pearance, from which the condition of by far the greater number of spots is most 

 undeniably deduced. The utmost therefore that can hence be alleged is, that 

 some few spots differ from all the rest, or from the multitude, and are not like 

 these excavations in the sun. Such cases or exceptions will not surely warrant 

 the conclusion, that no spot can be an excavation. This would be to reverse all 

 the rules of a just induction, by opposing to an irrefragable general argument 

 the force of one extremely limited and feeble. 



