120 



M. Janssen's FhotograpTis of the Sun. 



[May, 



There is thus a triple chance of success. First, because the irradiation 

 diminishes rapidly with the increase in diameter of the images, especially 

 if the exposure is lessened at the same time ; secondly, because the dimen- 

 sions of the details are enlarged, and therefore the details ought to be more 

 easily obtained ; finally, the defects of the sensitive surface are of less 

 relative importance. 



There is another circumstance which, in this case, is particularly favour- 

 able for obtaining very well defined images. In short exposures, the pho- 

 tographic spectrum is reduced to a very narrow band and the rays which 

 act belong to a little group almost monochromatic. 



The optical spectrum is on the contrary very extended, and therefore 

 photography may be expected to yield much better defined images than can 

 be obtained by eye observation. 



It is true that the photographic difiiculties increase with the size of 

 the image, but these diflRculties can be overcome by care and perseverance. 



M. Janssen has, therefore, since the commencement of these studies in 

 1874, constantly endeavoured to obtain the solar images larger and larger, 

 advancing from between 4 and 5 to 12 inches in diameter. 



At the same time as the dimensions were increased, the composition of 

 the sensitive surface and the mode of development were improved. The 

 development of the image ought to be very gradual, commencing with 

 iron and finishing with pyrogallic acid and silver. 



The greatest care must be taken in the focussing and in giving an even 

 exposure all over the plate. 



The length of exposure is very short. In summer it is between 

 To*o ^^^ "3 oV ^^ ^ second for the images of 12 inches diameter. 



In consequence of the very short exposure the development is slow, but 

 then the image appears in all its details, free from irradiation, and shows 

 the phenomena we have now to consider. 



The photographs show the solar surface covered with a fine general 

 granulation, the form, dimensions and arrangement of the granular 

 elements being very variable. In size they vary from some tenths of a 

 second to 3 or 4 seconds. The forms are circular or more or less elongated 

 ellipses, but often these regular forms are altered. 



The granulation is visible everywhere and does not seem to possess a 

 different composition towards the poles of the sun. The luminous power 

 of the granular elements considered separately is very variable. They 

 appear to be situated at different depths in the photospheric layer. 



The most luminous granulations only occupy a small portion of the 

 surface of the sun. 



But the most remarkable result, and which is due entirely to the aid 

 of photography, is the discovery of the photographic net-work. 



