160 Experimental Investigations 



reservoir ; and each reservoir has a little neck, through 

 which it is filled with water, and which after receives 

 the bulb of a cylindrical thermometer, that reaches com- 

 pletely across the inside of the box in the direction of 

 its diameter. 



The two reservoirs of heat, with their two lenses, are 

 firmly fixed in an open frame, which being movable in 

 all directions by means of a pivot and a hinge, the ap- 

 paratus is easily directed toward the sun, and made to 

 follow its motion regularly, so as to keep the solar 

 spectra constantly in the centres of the blackened faces 

 of the reservoirs. 



In order that the quantities of light passing through 

 the two lenses should be perfectly equal, a circular plate 

 of well-polished brass, in the centre of which is a circu- 

 lar hole 3! inches in diameter, is placed immediately 

 before each of the lenses. 



When the reservoirs of heat are placed at different 

 distances from the focuses of their respective lenses, the 

 diameters of the solar spectra which .are formed on the 

 blackened faces of the reservoirs are necessarily differ- 

 ent ; and as the quantities of light are equal, its density 

 at the surface of each reservoir is inversely as the square 

 of the diameter of the spectrum formed on that surface. 



Experiment No. I. In this experiment the reservoir 

 A was placed so near the focus of the lens, between 

 the lens and the focus, that the diameter of the solar 

 spectrum falling on it was only half an inch, or 6 lines, 

 while the reservoir B was advanced so far before the 

 focus that the spectrum was 2 inches in diameter, or 

 24 lines. 



As the quantities of light falling on both were equal, 

 the density of the light at the surface of the reservoir 



