14 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



for stud}-. They appear to be generally of a spherical shape, 

 and it is noticeable that larger grains seemed to be formed 

 of aggregations of smaller ones, the lesser grains being most 

 evidently spherical. An interesting account of one of Jans- 

 Ben's largest positives has been contributed to the American 

 Jow-nal of Science for April, 1878, by Professor Langley. 



In the Annuaire of the Bureau of Longitude for 1878 (p. 

 G89), M. Janssen gives a detailed account of his discovery of 

 the reticulated arrangement of the solar photosphere. The 

 paper is accompanied by a photograph of the appearances 

 described, which is enlarged threefold. M. Janssen says that 

 photographs less than 4 inches in diameter cannot satisfac- 

 torily show such details. The chief obstacle to the produc- 

 tion of photographs of the sun which should show the details 

 of the photosphere has hitherto been the photographic irra- 

 diation. As the granulations of the solar surface are in gen- 

 eral not greatly larger than l" or 2", the irradiation, which 

 sometimes is 20" or more, may completely obscure their char- 

 acteristics. This difficulty M. Janssen has overcome by en- 

 larorinor the ima^e and shortening the time of exposure. 

 In this way the irradiation is diminished as the diameters 

 increase, the dimensions of the details are increased, and 

 " the imperfections of the sensitive plate have less relative 

 importance." [This last advantage certainly holds good 

 only between limits.] 



Again, M. Janssen has noted that in short exposures the 

 photographic spectrum is almost monochromatic. In this 

 way it differs greatly from the visible spectrum, and to the 

 advantage of the former for this special purpose. The pure- 

 ly photographic difficulties are immensely augmented, how- 

 ever; but these have been surmounted by the care and per- 

 severance of M. Janssen and his assistant M. Anents. The 

 diameters of the solar photograrns have since 1874 been suc- 

 cessively increased to 4.7, 5.9, 7.9, and 11.8 inches. The fo- 

 cal adjustment varies greatly with the season, and even with 

 the time of day. The exposure is equal over all parts of the 

 sin race. In summer this exposure for the largest photo- 

 grams is less than 0.0005 s . The development of the pictures 

 is very Blow. These pictures show that "the solar surface is 

 covered with a line granulation. The forms and the dimen- 

 sions of these elements are very various. They vary in size 



