496 



Prof. Secchi on Solar Spots, 



mine, but Wilson's. To establish this theory, he relied upon the 

 appearances presented by a spot on its becoming visible at the 

 edge of the sun. or on its quitting it, because it then presented 

 itself with the penumbra diminished, and altogether wanting on 

 the side towards the centre of the disc. According to Wilson, 

 the penumbra would be formed by the declivity or slope of the 

 borders of the cavity itself. The case observed by AVilson was 

 thought tp be an exceptional one ; but, on the contrary, it is in 

 truth tolerably frequently observed, and I have very often noticed 

 its occurrence, especially in the case of spots of a circular form, 

 and which are not about to disappear. If this is so, it will then 

 be in our power to ascertain the depth of these cavities, and thus 

 to measure the thickness of the layer of the sun's photosphere. 

 A truly marvellous thing, that the human eye, which in its 

 natural state cannot direct itself on that luminary, should be 

 able at length to provide itself with instruments adapted to 

 measure the depth of that ocean of flame that invests it ! The 

 matter in itself is extremely simple. If we suppose there is, on 

 the spherical body Bjo A, any 

 cavity having the form of a trun- 

 cated cone, 71 o' n', in conformity 

 with this theory its bottom will 

 form the nucleus, and the walls 

 the penumbra. When the spot 

 is projected on the middle of the 

 disc it will appear circular; but 

 having moved by rotation to the 

 margin, its shape will be altered 

 to an oval, and it will finally 

 arrive in a position in which the 

 inclination or slope of the decli- 

 vity of the cone will become pa- 

 rallel to the visual ray, so that 

 the optical projection of the bot- 

 tom will touch the upper margin 

 of the cavity, as shown in the 

 figure. 



In this state of the matter the depth of the spot may be thus 

 arrived at. Let us suppose that the moment chosen to observe 

 it is that of the disappearance of the internal penumbra, and let 

 its distance to the edge of the disc be measured, as well also as 

 the breadth of the penumbra in the longer direction of the oval 

 in which this spot is projected ; this will readily give us the 

 depth sought. 



For if we draw the radius C n from the centre of the sun, we 

 have the angle BCn=C«K, whence as a consequence onp, 



