"BAILY'S BEADS." 241 



of the spots that they are huge cavities, great rifts of the luminous 

 surface of the sun, many thousands of miles in diameter, and proba- 

 bly some thousand miles deep. Let us suppose the case of a spot 

 say 2000 miles deep and 10,000 miles across (Sir W. Herschel has 

 measured spots of 50,000 miles diameter). When such a spot in the 

 course of the sun's rotation reaches that part which forms the visible 

 edge of the sun, it must, if rendered visible, be seen as a notch ; but 

 what will be the depth of such a notch ? Only about 1 -430th of the 

 sun's diameter. But the apparent depth would be much less as the 

 edge or riin of the spot next to the observer would cut off more or less 

 of its actually visible depth, this amount depending upon the lateral 

 or east and west diameter of the spot and its position at the time of 

 observation. 



Thus, the visible depth of such a notch would rarely exceed one 

 thousandth of the sun's apparent diameter, or might be much less. 

 The sun being globular, the edge which is visible to us is but our 

 horizon of his fiery ocean, which we see athwart the intervening stir- 

 face as it gradually bends away from our view. So small an indent 

 upon this edge would, under ordinary circumstances of observation, 

 be rendered quite invisible by the irradiation of the vast globular 

 surface of the glaring photosphere, upon which it would visually 

 encroach. 



If, however, this body of glare could be screened off, and only a 

 line of the sun's edge, less than one thousandth of his diameter, re- 

 main visible, the notch would appear as a distinct break in this 

 curved line of light. If a group of spots, or a great irregular spot 

 with several umbrae, were at such a time situated upon the sun's 

 edge, the appearance of a series of such notches or breaks leaving 

 intermediate detachments of the visible ring of the photosphere 

 would be the necessary result, and thus would be presented exactly 

 the appearance described as " Baily's beads." 



I have been led to anticipate a display of these beads during the 

 late eclipse by the fact that some days preceding it a fine group of 

 spots visible to the naked eye through a London fog were travel- 

 ling toward the eastern edge of the sun, and should have reached the 

 limb at about the time of the eclipse. The beads were observed by 

 the Rev. S. J. Perry just where I expected them to appear. I have 

 not yet learned on which side of the sun they were observed and 

 photographed by Lord Lindsay. 



Baily's first observation of the beads was made during the annular 

 eclipse of May 15, 1836. That year, like 1870, was remarkable for a 

 great display of sun-spots. As in 1870, they were then visible to the 

 naked eye. I well remember my own boyish excitement when, a few 

 weeks before the eclipse of 1836, I discovered a spot upon the red- 

 dened face of the setting sun a thing I had read about, and sup- 

 posed that only great astronomers were privileged to see. The rich- 

 ness of this sun-spot period is strongly impressed on my memory by 

 the fact that I continued painfully watching the dazzling sun, 

 literally " watching and weeping," up to the Sunday of the eclipse, 

 on which day also I saw a large spot through my bit of smoked glass. 



The previous records of these appearances of fracture of the thin 

 line of light are those of Halley, in his memoir on the total eclipse of 

 1715, and Maclauren's on that of 1737. Both of these correspond to 

 great spot periods ; the intervals between 1715, 1737, 1836, and 187G 



