VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 3C>9 



portions. To this end therefore he exhibits a figure drawn in just proportional 

 dimensions ; by which we may see how very small a portion of the sun's body 

 is made up of the luminous matter when supposed every where 3967 English 

 miles deep. 



For his own amusement too, Dr. W. says he had pursued this subject further 

 in the way of ocular proof, by a model of the sun and of the spots on his body 

 according to their proper dimensions. This he put into a convenient wooden 

 frame, and viewed it afar off" when set upon a stand, while the globe was turned 

 slowly round, and subtended an angle at the telescope equal to the apparent dia- 

 meter of the sun. By an object-glass micrometer he then took the distances from 

 the limb when the farthest umbra of different spots vanished, as also the distances 

 of the nuclei just when disappearing. The apparent subtense of the umbra next 

 the limb was also measured in this way, with the visible extension of some large 

 spots within the disk, when the extreme limits of the nearest umbra coincided 

 with the limb. In all these experiments, he says, the effect was very striking, 

 and the phenomena remarkably consonant to calculation, and to what he had 

 often seen on the real sun in the heavens. 



But to proceed ; what has now been insisted on at so much length concerning 

 the shallowness and the more gradual shelving of some few spots, will also apply 

 to another objection, which M. de la Lande views in a strong light. Here we 

 find quoted the great spot in 171Q, seen by M. Cassini ; and, for the 2d time, 

 that of June 3, 1 703, seen by M. de la Hire ; both which, on their arrival at 

 the limb, are said to have made an indentation or dark notch in the disk ; and 

 this phenomenon is mentioned as absolutely incompatible with spots being below 

 the surface. It is most true, says Dr. W. that if we look for any thing like this, 

 when the plane which coincides with the external boundary of the spot passes 

 through the eye, the way that M. de la Lande considers the matter, it must be 

 very large indeed before the disk could be perceived deficient by any dark seg- 

 ment. But may not a spot, even no larger than M. Cassini's, considered as an 

 excavation, make, in a manner very different from this, something like a notch ; 

 for, by the way, this phenomenon is not in the Mem. Acad, nor any where else, 

 that I know of, described with any sort of precision. After showing how this 

 may happen, Dr. W. adds, I do not imagine therefore that the phenomena of 

 notches in the disk, so inconsiderable and dubious as these seem to be, are by 

 any means a proof of projecting nuclei, or that they are not reconcileable to 

 spots being depressions in the sun. A large shallow excavation, with the sloping 

 sides or umbra darker than common, may be more or less perceptible at the 

 limb. 



In reasoning concerning the nature of the spots, and particularly about their 

 3d dimension or depth, the only arguments which are admissible, and which 



vol. xv. 3 B 



