444 Dr. Herschel's Experiments on the solar, and 



lishment of the contents of the preceding one ; for the refran- 

 gibility of heat rays could not be thus corrected, were the sines 

 of refraction not in a constant ratio to those of incidence. 



23d Experiment. In Burning-glasses, the Focus of the Bays of 

 Heat is different from the Focus of the Rays of Light. 



I placed my burning lens, with its aperture reduced to three 

 inches, in order to lessen the aberration arising from the sphe- 

 rical figure, in the united rays of the sun; and, being now 

 apprised of the different refrangibility of the rays of heat, and 

 knowing also that the least refrangible of them are the most 

 efficacious, I examined the focus of light, by throwing hair- 

 powder, with a puff, into the air. This pointed out the mean 

 focus of the illuminating rays, situated in that part of the pencil 

 which opticians have shewn to be the smallest space into which 

 they can be collected. That this may be called the focus of light, 

 our experiments, which have proved the maximum of illumi- 

 nation to be situated between the yellow and green, and there- 

 fore among the mean refrangible rays of light, have fully estab- 

 lished. The mean focus being thus pointed out by the reflection 

 of light on the floating particles of powder, I held a stick of 

 sealing wax i",6, or four beats of my chronometer, in the con- 

 tracted pencil, half an inch nearer to the lens than the focus. 

 In this time, no impression was made upon the wax. I applied 

 it now half an inch farther from the lens than that focus ; and, 

 in 8-tenths of a second, or two beats of the same chronometer, 

 it was considerably scorched. Exposing the sealing wax also 

 to the focus of light, the effect was equally strong in the same 

 time ; from which we may safely conclude, notwithstanding the 

 little accuracy that can be expected, for want of a more proper 



