A. MATHEMATICS AND ASTRONOMY. 43 



vention he thinks it possible that an entirely new branch of 

 solar physics may be created. He causes 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 MeissePs 

 double iodide of copper and mercury. When the wave of 

 heat, passing through the carbon and the paper, has warmed 

 the thin film of iodide to the temperature of 70 Centigrade, 

 this substance is blackened ; and if, beginning with a very 

 small aperture of the telescope, we gradually increase it un- 

 til we obtain the smallest area of blackened iodide that can 

 be produced with a well-defined contour, we thus obtain a 

 determination of the area of maximum temperature on the 

 solar disk. On using a larger aperture of the object-glass 

 of the telescope, a larger surface of blackened iodide is form- 

 ed, the new area being bounded as before by a well-defined 

 isothermal line ; and by repeating this process maps are ob- 

 tained of the isothermals of the solar disk. An exposure 

 of about twenty minutes is required to obtain these thermo- 

 graphs, which are sufficiently permanent to allow one to 

 trace accurately their isothermal contours. But other sub- 

 stances exist which are more suitable than the iodides for 

 producing permanent thermographs. Professor Mayer states 

 that, as far as he has at present applied this method, he con- 

 cludes that there exists on the solar disk an area of sensibly 

 uniform temperature and of maximum intensity. This area 

 of maximum temperature is of variable size. It is in motion 

 on the solar image. It is surrounded by well-defined iso- 

 thermals. The general motions of these isothermals follow 

 the motions of the central maximum area, but they have 

 also their independent motions. American Journal of Sci- 

 ence, July, 1875, 50. 



A NEW METHOD OF COMPUTING PLANETARY PERTURBATIONS. 



The immense amount of labor that has, for a hundred 

 years past, been spent upon the computation of the mutual 

 perturbations of the planets, and the great amount of time 

 and ingenuity employed by mathematicians in devising the 

 methods of special perturbations and mechanical quadra- 

 tures, would justify a prolonged notice of the new method of 

 computing special perturbations developed in a recent me- 

 moir by Professor Gylden, of Stockholm, and which has been 



