of the prismatic Colours to heat and illuminate Objects. 257 



sensibility. The scales of all were properly disengaged from 

 the balls. 



I now placed the stand, with the framed pasteboard and the 

 thermometers, upon a small plain board, GH ; that I might be 

 at liberty to move the whole apparatus together, without de- 

 ranging the relative situation of the different parts. 



This being done, I set a prism, moveable on its axis, into the 

 upper part of an open window, at right angles to the solar ray, 

 and turned it about till its refracted coloured spectrum became 

 stationary, upon a table placed at a proper distance from the 

 window. 



The board containing the apparatus was now put on the 

 table, and set in such a manner as to let the rays of one colour 

 pass through the opening in the pasteboard. The moveable 

 frame was then adjusted to be perpendicular to the rays coming 

 from the prism ; and the inclined planes carrying the three 

 thermometers, with their balls arranged in a line, were set so 

 near the opening, that any one of them might easily be ad- 

 vanced far enough to receive the irradiation of the colour which 

 passed through the opening, while the rest remained close by, 

 under the shade of the pasteboard. 



By repeated trials, I found that Dr. Wilson's No. 2 and 

 mine, always agreed in shewing the temperature of the place 

 where I examined them, when the change was not very sud- 

 den ; but that mine would require ten minutes to take a change, 

 which the other would shew in five. No. 3 never differed 

 much from No. 2. 



1st Experiment. Having arranged the three thermometers in 

 the place prepared for the experiment, I waited till they were 

 stationary. Then, advancing No. 1 to the red rays, and leaving 



LI 2 



