50 A. M. Mayer—Thermographs of Isothermal Lines. 
Arr. XIT.—The Discovery of a method of obtaining Thermographs 
a en —— Lines of the Solar Disc; by ALFRED M. 
On June the 5th, 1875, I devised a method for obtaining the 
isothermals on the solar disc. As this process may create an 
entirely new branch of solar physics, I deem it proper that I 
should give a short account of it in order to establish my claim 
as ae 
he American Journal, July, 1872, I first showed how 
oe en with great precision, trace the progess and determine 
the boundary’ of a wave of conducted heat in crystals, by coat- 
ing sections of these bodies with Meusel’s double iodide of cop- 
per and mercury, and observing the blackening of the iodide 
where the 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 disc as we formerly did 
on the crystal sections. 
The method of proceeding is as follows: Beginning with an 
aie of object-glass which does not give sufficient heat in 
of the solar image to blacken the iodide, I gradually 
increase athe aperture until I have obtained that area of black- 
ened iodide which is the smallest that can be produced with a 
well defined contour. 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 exposing for about 20 minutes the 
surface of iodide to the action of the heat inclosed in an iso- 
thermal, I have obtained thermographs of the above areas; 
which are grap etd jeepers ad allow one to trace accurately 
their isothermal contours. are other substances, how- 
ever, which are ane Tatabie than the iodide for the produc- 
tion of permanent thermographs. 
The contours of the successively blackened areas on the iodide 
are isothermals, whose successive thermometric values are iD- 
versely as the successively i increasing areas of aperture of object 
glass which eeenrely produced foes. 
As far as w observations have any weight, the following 
appear to be i. y Oeinsithe already made of this new met 
(1) There exists on the solar image an area of sensibly uniform 
temperature and of maximum intensity. (2) This area of maxi- 
