486 THE POPULAR SCIENCE MONTHLY. 



constitution is, that the surface of the sun presents the aspect of a net- 

 work, the web of which is formed of strings of more or less regular 

 grains, with here and there elongated bodies drawn out in all direc- 

 tions. An attentive study of these curious phenomena leads us to a 

 very simple explanation of them. 



The stratum of luminous matter to which the sun owes its power 

 of radiation is, as we know, very thin. If this sti'atum was in a state 

 of perfect equilibrium, the fluid matter of which it is constituted would 

 form a continuous envelope around the nucleus of the sun ; and the 

 granular elements being confounded together, the solar surface would 

 have everywhere a uniform brightness. But the ascending currents, 

 of which the eruptions of metallic vapors and the hydrogen -protuber- 

 ances are evidences, rupture the fluid stratum which is tending to form 

 at a great number of points. It is then broken up and divided into 

 more or less considerable fragments. Wherever the perturbing forces 

 leave the elements of the photosphere in a state of relative repose, 

 they take a more or less pronounced globular form. At those points, 

 on the other hand, which are the seats of ascending currents, these 

 elements give evidence in their aspect of the violence of the actions to 

 which they are subjected. Hence the variable forms of the elements 

 of the photosphere, concerning which there has been so much discus- 

 sion. Hence, also, the explanation of that net-work-like structure of 

 the solar surface which has been revealed by photography. 



These images also show the enormous difference between the lumi- 

 nous power of the elements of the photosphere and that of the medium 

 in which they float, which seems quite dark by the side of them. A 

 result of this constitution is, that the radiating power of the sun will 

 be affected according to the number and brightness of these elements. 

 The spots, then, can no longer be regarded as the principal element in 

 the variations of the solar radiation ; a new factor, the action of which 

 may be preponderant, must hereafter be added to them. 



These photographs permit another study, which promises results 

 of extreme importance the study of the motions which the granular 

 elements take on under the action of the forces that rumple the photo- 

 sphere. For the study of these motions, successive images of the 

 same point on the surface of the sun are taken at very brief intervals 

 with the photographic revolver. A comparison of the images demon- 

 strates that the matter of the photosphere is animated by movements 

 of the violence of which our terrestrial phenomena can convey only a 

 very feeble idea. 



Following the example of spectrum analysis, photography is mak- 

 ing a circuit of the heavens. The year 1881 witnessed the first tak- 

 ing of the photograph of a comet, with a considerable portion of its 

 tail. This picture has revealed some curious particulars of structure 

 and has permitted a number of photometric measurements, the most 

 notable of which is one showing that the tail, notwithstanding the 



