160 Intelligence and Miscellaneous Articles. 



wave of conducted heat reaches 70° C. If we cause the image of 

 the sun to fall upon the smoked surface of thin paper while the 

 other side of the paper is coated with a film of the iodide, we 

 may work on the solar disk as we formerly did on the crystal 

 sections. 



The method of proceeding is as follows : — Beginning with an 

 aperture of object-glass which does not give sufficient heat in any 

 part of the solar image to blacken the iodide, I gradually increase 

 the aperture until I have obtained that area of blackened iodide 

 which is the smallest that can be produced with a well-defined con- 

 tour. This surface of blackened iodide I call the area of maximum 

 temperature. On exposing more aperture of object-glass, the 

 surface of blackened iodide extends and" a new area is formed 

 bounded by a well-defined isothermal line. On again increasing 

 the aperture another increase of blackened surface is produced 

 with another isothermal contour ; and on continuing this process I 

 have obtained maps of the isothermals of the solar image. By ex- 

 posing for about 20 minutes the surface of iodide to the action of 

 the heat inclosed in an isothermal, I have obtained thermographs 

 of the above areas, which are sufficiently permanent to allow one 

 to trace accurately their isothermal contours. There are other 

 substances, however, which are more suitable than the iodide for 

 the production of permanent thermographs. 



The contours of the successively blackened areas on the iodide 

 are isothermals, whose successive thermometric values are inversely 

 as the successively increasing areas of aperture of object-glass which 

 respectively produced them. 



As far as the few observations have any weight, the following 

 appear to be the discoveries already made by this new method : — 

 (1) There exists on the solar image an area of sensibly uniform 

 temperature and of maximum intensity. (2) This area of maxi- 

 mum temperature is of variable size. (3) This area of maximum 

 temperature has a motion on the solar image. (4) The area of 

 maximum temperature is surrounded by well-defined isothermals 

 marking successive gradations of temperature. (5) The general 

 motions of translation and of rotation of these isothermals appear 

 to follow the motions of the area of maximum temperature which 

 they inclose ; but both central area and isothermals have indepen- 

 dent motions of their own. 



On projecting the enlarged image of a sun-spot on the blackened 

 surface and then bringing a hot-water box, coated with lamp-black, 

 near the other side of the paper, one may develop the image of the 

 spot in red on a dark ground. A similar method probably may 

 serve to develop the athermic lines in the ultra-red region of the 

 solar and other spectra. — Sillirnan's American Journal, July 1875. 



