NATURAL PHILOSOPHY. 143 



alent to that of the whole image of the sun in a refractor of thirty-two 

 inches focal length, and of two inches aperture. This may usually be 

 employed without injury to the dark glasses, and the field is quite large 

 enough for sweeping over the sun's disc in searching for spots or other 

 phenomena. Tn careful scrutinies, the suitable aperture must be 

 employed. In some very large spots, the nuclei alone may thus be 

 examined, without any disturbance frem the brio-lit surface. " By this 

 mode of observation I have ascertained the existence of a stratum of 

 comparatively faint luminosity, which, as far as I know, has not been 

 previously noticed. For this I would propose the appellation of the 

 cloudy stratum. Its appearance gives the impression of considerable 

 depth below the second luminous stratum which forms the shallow, or 

 penumbra, usually seen round the nucleus of a spot ; and from all the 

 examinations I have hitherto made, it seems to me probable that it is 

 not self-luminous, but of such a nature as to absorb a vast quantity of 

 light, and reflect very little. Its faint illumination is rarely uniform, 

 presenting rather a mottled or cloudy surface ; and occasionally some 

 small patches are very decidedly more luminous than the rest, though 

 still incomparably less bright than even the stratum forming the penum- 

 bra ; from which it also differs essentially in being destitute of the 

 striated or ridged appearance so frequently presented in that stratum. 

 In all spots of considerable size, and in many small ones, a black 

 opening is perceivable in the cloudy stratum. In no instance have I 

 perceived any light in these openings which exceeded the illumination 

 of the earth's atmosphere by the sun's rays. It is obvious that any 

 degree of light inferior to this cannot be rendered visible by any con- 

 trivance we "can employ ; just as the red projections from the sun's 

 border cannot be seen except when the solar illumination of our atmos- 

 phere is nearly extinguished by the intervention of the moon. In 

 order to obtain a measure of this illumination of the atmosphere, and 

 to ascertain the limit, which it sets to our researches, I have usually 

 directed the telescope, with a very small field, to the sky close 

 to the sun's edge, using the lightest shade of darkening glass 

 which my eye could comfortably bear. Then, using the same shade, 

 the telescope is directed with the smallest diaphragm to the dark part 

 of a large spot. In this way the cloudy stratum will in general become 

 visible, frequently occupying by far the greater part of what has been 

 hitherto considered the nucleus of the spot, and imagined to be, in fact, 

 the body of the sun itself. A portion of it, however, will commonly 

 be found to appear perfectly black, whence we may conclude that, if 

 luminous at all, it is less so than our own atmosphere when illuminated 

 by the direct rays of the sun. To this black part only the appellation 

 of nucleus appears to be strictly applicable. A remarkable instance 

 of rotatory motion was observed in a spot which was sketched on Jan- 

 uary 17 and January 23. The rotation was not of the smaller round 

 the 'larger portion. The whole spot had rotated round the small black 

 nucleus." On examining the surface of the sun carefully, using a 

 very small field, Mr. Dawes is persuaded that " the apparent rapid 

 fluctuation of the porous structure is not real, but the effect of dis- 

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