1824.] 



Mr. Powell on Solar Light and Heat. 



83 



the surface of the bulb, and exposing either the plain or coated 



side to the same intensity of heat. The ratio - maybe obtained 



by heating two bulbs completely coated to the same point, and 

 observing their rates of cooling. 



The case (C) is the same as that investigated in the Phil. 

 Trans. 1800, No. 19, note, p. 447. 



(29.) Comparison of the Focal Effect of Two Lenses on a Ther- 

 mometer coated with Indian Ink. 



In order to proceed to this comparison, we must first observe, 

 that when in the formula we take a . d", it is on the supposition 

 that parallel rays impinge on a spherical surface. With the 

 focus this is not the case, and from the convergence of the rays, 

 as well as from their greater intensity at the outer edge, we may 

 in this case assume, without fear of error, that a = the area of 

 the section of the rays impinging, and thus apply the formula. 

 Thus we have the following data : 



From the above experiments, r, = 10, r = 31 : it is also 

 evident, that/) = p t and k = /;,. By measurement, the diame- 

 ters of the focal disks were : 



Lens I . 



0-25 inch 

 .'. the areas 0-049 sa a 



Lens 2. 

 0-16 inch 

 0-021 = a, 



d = 0-45 

 Hence s - 

 and we have to apply the case of the formula (R) 



ft _ 31 5S7_ 21 160 l 



= 10 



. s = -636. 



«, = '61o $ — a = -587, 





A, 10 " ■ 615 " 49 ~ 301 ~~ 1-80 J P* 8 *V m 



(30.) In order to calculate the respective intensities of light, 

 or number of rays collected in the locus of each lens, we may 

 easily proceed by the well-known theorem, 



