1878.] M. Janssen's I*lioto graphs oftlie Sun. 119 



Capt. Waterhouse exhibited a photograph o£ part of the sun's disc, 

 obtained by M. Janssen at the Observatory of Meudon, near Paris. He 

 said : The photograph I have the pleasure of exhibiting is one published in 

 the Annuaire du Bureau des Longitudes, Paris, for 1878, and represents a 

 small portion of the solar disc enlarged from one of M. Janssen's negatives 

 taken on the 10th October 1877, at 97*. 36w. On M. Janssen's negative the 

 diameter of the sun's disc was 305 onm., or about 12 inches, and it has been 

 enlarged three times, consequently the full diameter of the disc as repre- 

 sented in part in the photograph is 0«^.•92, or about 36i- inches. 



The photograph is accompanied by an explanatory note by M. Janssen, 

 in which he lays down the principles that have guided him in making 

 these photographs, and the following is a brief abstract of it. 



Up to the present time photography, considered as a means of describ- 

 ing the surface of the sun, has remained much inferior to eye observations 

 with large instruments. 



Photographs on which the sun's image is not more than 4 or 5 inches 

 in diameter cannot show the structure of the photosphere, but this is indis- 

 pensable towards making any progress in solar knowledge. 



The study of the solar spots, which for the last two centuries and a 

 half has furnished almost the only data on the constitution of the photo- 

 sphere, seems now almost worked out, or, at least, it ought to be supple- 

 mented henceforth by the study of the photosphere itself. 



The study of the photosphere by eye observations is attended with 

 great difSculties, the chief of which is the impossibility of clearly recognis- 

 ing the form of the granulations in the midst of the flaming photosphere, 

 or to measure them and, still less, to identify them in order to follow 

 their changes. 



Consequently it would be an immense advance in solar knowledge if 

 photography could give us images of the sun's surface, showing the details 

 sufficiently clearly to permit this study ; it would also be a foundation of 

 future progress and discoveries. 



The solution of this problem has occupied M. Janssen from the time 

 that he commenced these solar photographic observations. 



On considering the conditions under which solar photographs had 

 hitherto been taken, M. Janssen found that the principal cause which pre- 

 vented the details of the solar surface from appearing on the photographs 

 was photographic irradiation, by which the images of bright objects are 

 enlarged ; and it is therefore evident that if the details of the granulation 

 of the solar surface are smaller than the amount of this irradiation, it will 

 be impossible to obtain them with any sharpness. 



M. Janssen considered that the solution of the difficulty was in en- 

 larging the image, combined with a diminution in the time of exposure. 



