498" Prof.' Secchi on Solar Spots, 



of the earth, or in other wordsj somewhat more than a third of 

 that radius. * . 



Such a depth (equal tc about 1330 English statute miles) may 

 appear too inconsiderable^ since it would not form a coating of 

 0'16 of an inch on a glo%.df 39'37 inches radius English mea- 

 sure ; nevertheless it cannotbe very far from the truth. Not that 

 1, however, maintain one cbb. found a certain opinion upon this 

 point on this sole measuremeut; for it is without doubt necessary 

 to repeat such observatioiis'ii-equently, inasmuch as indisputably 

 the depth of the photosphere is not the same in all parts ; yet I 

 am convinced that the "stratum cannot be of any great altitude, 

 because the spots never, show themselves devoid of a penumbra 

 in the inner portion, except when they have arrived extremely 

 close to the edge. 



We have found that the inclination of the declivity is but very 

 inconsiderable ; but in the spots about to disappear this element 

 must vary to a great extent, and diminish, as is shown by the 

 fact that, at the time the penumbra is barely to be distinguished 

 from the other parts, its limits are very ill defined, so that under 

 a high magnifying power it can hardly be recognized. It is a 

 curious but certain fact, that the inequality of light between the 

 general ground of the sun and the penumbras diminishes consi- 

 derably with the power of the eye-piece, the contrast being far 

 more marked with lower powers, — a remark which likewise holds 

 good with respect to the belts of Jupiter and Saturn. At any 

 rate the diminution in this contrast, when the spot is about to 

 disappear, is an additional proof that Wilson's theory is the most 

 probable, — that is to say, that the penumbra is due to the dimi- 

 nution of light arising from the different inclination of the surface 

 from which it emanates with respect to the eye of the observer. 



Many have objected to Wilson's theory, that so great a dimi- 

 nution could not take place from this sole cause ; ■ but if the 

 theory assigned by him is not satisfactory in every respect, I 

 think that, from the recent observations of ourselves and others, 

 no doubt can remain with regard to the fundamental point when 

 attention is given to another consideration which I will now enter 

 into. We have repeated several times, that the penumbras are 

 formed of streamlets and divided into extremely fine filaments, 

 each of which is in itself of almost equal brightness to tlie general 

 photosphere, but that, when seen confusedly mixed up together 

 with intervals and obscure lines under moderate powers, they 

 produce the illusion of a half-tint, as in engravings on copper. 

 This is the case; and subsequent observations have borne out 

 what we stated some years since. But there is, in addition, yet 

 another cause not sufficiently insisted upon or known ; and it is 

 as follows : — 



