210 ON THE EXTERNAL APPEAEAXCE OF THE SUN's DISK. 



be throws gome doubt upon tbe circumstance of a continual .change in the state 

 of these pores. He says that he has explored and considered the surface of the 

 sun with the greatest care, reducing the diaphragm of his telescope to small 

 openings of from 20 to 60" diameter, and availing himself of a greater degree of 

 enlargement than is generally to be commanded; that he has frequently retained 

 the same shining masses, with the included pores, under observation for not less 

 than two hours, but has seldom at any time observed a change, even Avith a 

 magnifying power of 4,600 fold. He adds that the disturbances which so often 

 take place in the atmosphere are sufficient to produce the belief that a constant 

 change is going on in the objects we observe, and that the eye, when confined 

 to so narrow a field, soon grows weary, and the vision becomes embarrassed. 



To this comparative repose, however, there is one fact which forms an excep- 

 tion, for when our observation is directed to the immediate neighborhood of the 

 spots, these are found to enlarge or diminish with great rapidity. It is chiefly 

 under these circumstances that the lustrous masses assume the lengthened shape 

 before spoken of; but these changes are the most vivid when these bright 

 masses extend themselves by a rapid movement across the abyss, and thus form 

 those dazzling bridges which often proceed from the principal spots. The point 

 from which such a movement proceeds is frequently indicated by an accumula- 

 tion and bending of the greater axis of one of the lengthened masses in the 

 direction of the movement. Here M. Dawes shares the opinion of Sir John 

 Herschel respecting the appearance of chemical precipitation, and refers to it the 

 cause of these phenomena. 



Wherever this appearance occurred, M. Dawes narrowed his observation to 

 the edge of the spots, and embracing but a small field, examined with attention 

 the formation of the first part of the bridge. The bright masses presented the 

 appearance of haums of straw, lying nearly all in the same direction and but few 

 oblique to the line of the bridge, the sides of which seemed notched, on account 

 of the unequal length of the parts of which it was formed. It may be remarked 

 that these bridges are always constituted by bright stripes or lines which pro- 

 ceed from the outer envelope and are projected to the half-shadows, without 

 mingling with the under and less brilliant strata. M. Dawes at least has never 

 found it otherwise. The light of these stripes has always been of such intensity 

 that the lines formed by the bridges, however narrow, rendered it impossible to 

 distinguish with the eye the spot from the shadow. 



As regards investigations respecting the origin or cause of the sun-spots, M. 

 Dawes advises observers to direct particular attention to the dark nucleus which 

 presents itself in the shadow of the most symmetrical spots. For twelve years 

 has he been made sensible of the inconvenience arising from the application of 

 the same name to different objects and the failure to distinguish the shadow 

 from the nucleus. This admonition has unfortunately been little heeded ; in 

 the description of the spots the shadow is constantly confounded with the true 

 nucleus. M. Dawes the more insists upon this point, because his own observa- 

 tions, have satisfied him that the presence or absence of the nucleus is competent 

 to determine the origin of the spots, or at least to throw a strong light upon the 

 question, and that the origin of the spots in which the nucleus occurs is alto- 

 gether different from that where no such phenomenon appears. 



To avoid all confusion it is of course indispensable to denominate each sev- 

 eral thing by its true name. From the diligent observation of Dawes it results 

 that the shadow. under the half-shadow is not the sun's nucleus, and that this 

 nucleus sometimes appears in the centre of the shadow. These two essentially 

 different facts should never be lost sight of. 



