368 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



visual ray, the extreme nearness to the limb would prevent our forming any cer- 

 tain judgment of either. 



From this last example, therefore, it appears manifest that a spot, answering 

 to the description and conditions there mentioned, or one a little more shallow, 

 would approach the limb, and finally go off the disk, without that peculiar change 

 of the umbra on one side, which is so obvious on common occasions, notwith- 

 standing it were an excavation, whose nucleus or bottom is so many miles below 

 the level of the surface. 



In 4 cases, stated by Dr. W. the distance of the remotest part of the nucleus 

 from the sun's limb when the visual ray coming trom it is just interrupted by the 

 lip of the excavation, or, in other words, the distance of the nucleus from the 

 limb when it is totally hid was also computed. These distances are as follow : 



Case 1 1 6'.g3 Case 3 4".70 



2 8 .QO 4 2 .70 



and it is remarkable from the last two, how very near the limb a shallow spot, of 

 not more than 40" in diameter, may come before the nucleus wholly disappears. 



Perhaps it may be urged, that very shallow spots ought always to be known 

 from the rest, and to discover themselves, by a surrounding umbra very narrow 

 compared to the extent of the nucleus; but we know far too little of the qualities 

 of the luminous matter, and of the proximate causes of the spots, to say any 

 thing at all on a point of this kind. The breadth of the umbra is, as assumed 

 in the computations, commonly about equal to that of the nucleus, though 

 sometimes it varies more or less ; but how far these relative dimensions indicate 

 depth or shallowness must be expounded only by observation, and not by any 

 vague or imperfect notions of the nature and constitution uf the sun. The men- 

 tion of a pit or hollow or excavation several thousands of miles deep, reaching to 

 that extent down through a luminous matter to darker regions, is ready to strike 

 the imagination in a manner unfavourable to a just conception of the nature of 

 the solar spots as now described. On first thoughts it may look strange, how the 

 sides and bottom of such vast abysses can remain so very long in sight, while by 

 the sun's rotation they are made to present themselves more and more obliquely 

 to our view. But when it is considered, how extremely inconsiderable their 

 greatest depth is, compared to the diameter of the sun, and how very Mde and 

 shelving they are, all difficulties of this sort will be entirely removed. Dr. W. 

 here objects to an unfair or incorrect sketch of a figure to represent a case of a 

 spot by M. de la Lande, and adds, that as his design, on the present occasion, 

 is to write and to explain matters in a popular way, rather than to astronomers, 

 it will be proper to assist the conceptions of those who are but little versed in 

 mathematical principles by such diagrams as will show things in their just pro- 



