1824.] Mr. Powell on Solar Light and Heat. 



89 



Hence - = rr« 



l-i 



Hence on the same principles as before (r and r, being now 

 the respective rates of cooling), we have 



* = - — ; and since by experiment 



r, rf, 



10 



IT 



, d 1000 



and 5; -"sis' 



We obtain | = ^°, 



Whence taking £-£ = ^, we get 



Here again if when the focus was thrown on the bulb, it was 

 encompassed by a sort of penumbra of a heating effect, this 

 being of the same nature as simple heat, acted on the black and 

 white surfaces in the inverse ratio of the diameters, and, there- 

 fore, tended by the addition of very small quantities in that ratio 

 to each of the terms of the ratio, to increase it, though probably 

 the effect was altogether too small to be perceptible. 



(39.) We now proceed to compare these heating effects with 

 the intensities of light absorbed by the black and white surfaces. 

 In the first instance, I attempted roughly to estimate the pro- 

 portions of light reflected, and thence reciprocally absorbed by 

 black and white surfaces in the following manner : On a red 

 "round were fixed a black and a white small circular disk ; also 

 l wo similar disks on a blue ground. Remaining at a fixed dist- 

 ance from them, having first darkened the room completely, I 

 increased by degrees the aperture of a sliding shutter, t.ll nrst 

 the white disk, and then the black, became visible. 1 his was 

 repeated several times, and the mean ratio of the size ot th e 



