on the terrestrial Rays that occasion Heat. 47 1 



placed from the prism, must be such as will allow the rays to 

 diverge sufficiently for the required colour to fill the transmit- 

 ting holes ; and the balls of the thermometers placed under them 

 ought to be less than these holes, that the projected rays may 

 pass around them, and shew their proper adjustment. The dia- 

 meters of mine, used for this purpose, are s£ tenths of an inch. 



g/^tb Experiment. 



I placed my apparatus at five feet from the prism, and so as 

 to cause the red-making rays to fall between the parallel lines, 

 in order to find what heat-making rays would come to the ther- 

 mometer along with them. 



Red rays. Bluish-white glass. 



Therm. A. Therm. B. 



°' 75i ■ 75 



5 77i 76! ... 2: ^ = ,625 



From this experiment it appears, that when a thousand red- 

 making rays fall on each transmitting hole, 375 of them, if they 

 also be the heat-making rays, are stopped by the bluish-white 

 glass which covers one of these holes; or," what requires no 

 other proof than the experiment itself, that 375 rays of heat, 

 of the same refrangibility with the red rays, are intercepted by 

 this glass. 



95th Experiment. 



Red rays. Flint glass. 



75i - 75i 



771 76f • • 14 : H = ,857 



This glass stops only 143 rays of heat which are of the same 

 refrangibility with the red rays. 



3P2 



