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





No. 5. 



No. 1. 





Sun, 



Bluish-white glass 



O' 



67 



67 



1 



68£ 



m 



2 



7o* 



%i 



3 



7H 



70 



4 



72I 



70^ 



5 



73 



7ii...6: 4i = ,75o 



Here the sun communicated, in 5 minutes, 6 degrees of heat 

 to the thermometer No. 5, which was openly exposed to its 

 action ; while, in the same time, No. 1 received only ^\ degrees 

 by rays transmitted through the bluish-white glass : then, as 

 6 : 4£ : : 1 : ,750. This shews plainly, that only | of the inci- 

 dent heat were transmitted, and therefore that £ of it was 

 intercepted by the glass. 



I shall here, as well as in the following experiments, point 

 out the difference between heat and light, in order, as has been 

 mentioned before, to lead to an elucidation of our last discus- 

 sion. To effect this, therefore, I have ascertained, with all the 

 accuracy the subject will admit of, the quantity of light trans- 

 mitted through such glasses as I have used ; but, as it would 

 here interrupt the order of our subject, I have joined, at the 

 end of this Paper, a table, with a short account of the method 

 that has been used in making it, wherein the quantity of 

 light transmitted is set down ; and to this table I shall now 

 refer. 



To render this comparative view more clear, we may sup- 

 pose always 1000 rays of heat to come from the object: then, 

 750 being transmitted, it follows, that the bluish-white glass 

 used in our experiment stops 250 of them ; and, by the table at 



