the terrestrial Rays that occasion Heat. 531 



required for the purpose ; for, by attending only to brightness, 

 it has often happened to me, that both objects appeared at last 

 of the same colour ; which proved to be some mean between 

 the two appearances considered separately. 



Some glasses stop so much light, that it will be advisable to 

 take them by the assistance of an intermediate one. Thus, in- 

 stead of comparing the open vane directly to a red glass, I 

 settle first the ratio of the violet one to that vane ; then, taking 

 the ratio of the red to the violet, and compounding these two 

 ratios, the result will be more accurate. The reason for this 

 will be easily comprehended, when the construction of the appa- 

 ratus is considered. For a red glass, immediately compared to 

 the open vane, would require its object to be brought extremely 

 near the lamp, while the other must remain at a very great 

 distance. This would occasion a considerable difference in the 

 angles, both of incidence and of reflection, between the rays 

 falling on one vane, and on the other. But, by dividing the 

 observation into two operations, we avoid the errors that might 

 be occasioned by the former arrangement. 



In the following table, the first column contains the names of 

 the different substances through which light has been trans-? 

 mitted. The second column shews the transmission of light, 

 expressed in decimal fractions ; or the proportion which it bears 

 to the whole incident light considered as unity. An arithme- 

 tical complement to this fraction, or w r hat it wants to unity, 

 will therefore give us the proportion of light which is stopped 

 by each of the substances contained in the first column ; and 

 that quantity multiplied by 1000 is placed in the third column. 



