METHODS IN MODERN PHYSICAL ASTRONOMY. 485 



the aid and the applications which physical astronomy can expect 

 from it. 



The first application which was made of photography to the study 

 of the sky was in France, whatever may be said about it. The first 

 image of a fixed star upon the daguerrean plate was that of the sun, 

 and it was taken by the authors of the admirable processes for meas- 

 uring upon the earth the velocity of light MM. Fizeau and Fou- 

 cault. Shortly afterward, images of the moon were obtained in the 

 United States. These labors were followed by others, of which the 

 sun and the moon were also the objects. Beautiful proofs of lunar 

 photographs have been taken by Mr. Warren de La Rue and Mr. 

 Rutherfurd. Photographs of the sun are taken regularly in many 

 observatories, as aids in the study of the spots and faculae of that star. 

 More recently, Mr. Rutherfurd and Mr. Gould have begun to make 

 celestial maps, and photographs of the nebula in Orion have been ob- 

 tained in New York (by Mr. Draper) and at Meudon. 



These works are all very important ; they bear upon the primary 

 object of astronomical photography, that of obtaining durable and 

 trustworthy images of the stars and the phenomena produced upon 

 them, available for further studies and measurements. Hitherto, ob- 

 servers had only memory, a written description, or a drawing, to de- 

 pend upon for the preservation of the recollection of a phenomenon. 

 Photography substitutes for this the material image of the phenom- 

 enon itself. It is an admirable artifice, which in a certain manner 

 prevents the extinction of the phenomenon and its passage to among 

 the things that were, and keeps it always with us for examination or 

 study. Important as these results may be, the latest labors of which 

 photography has been the object, especially in what concerns the sun, 

 have demonstrated that the method may be employed as a means of 

 making discoveries in astronomy. 



The large solar images which have been obtained in the latest 

 years at Meudon have revealed phenomena of the surface of the sun 

 which our largest observatory instruments could not have shown, and 

 which open a new field of studies. By their aid we can at last dis- 

 tinguish the real form of those elements of the photosphere, respecting 

 which so many different and contradictory assertions have been made. 

 The elements in question are composed of a fluid substance readily 

 obedient to the action of external forces. At points of relative calm, 

 the matter of the photosphere assumes forms more or less approaching 

 the spherical, and its aspect is that of a general granulation. Where- 

 ever, on the other hand, currents and more violent movements of the 

 matter prevail, the granular elements are more or less drawn out, and 

 present aspects suggesting the forms of grains of rice, willow-leaves, 

 or veritable threads. The regions, however, where the photosphere is 

 more agitated, are limited spots, and the granular form is generally 

 observed in the intervals between them. The result of this peculiar 



