154? Experimental Philosophy. [Lecture II. 



came from the bottom going upward, and that 

 which came from the top downward. The figure 

 given there is flat, but it must be supposed 

 spherical, the glass being represented edgeways. 

 If so, therefore, the collected bundle of rays, 

 passing through the glass, unite and form a cone, 

 or a figure like a candle extinguisher, the bottom 

 of which is at the glass, and the point at F. This 

 point, as I once before had occasion to mention, 



. is called the focus of the glass. From a calcula- 

 tion in geometry, we learn that the distance from 

 this point is always equal to the diameter of the 

 circle which the glass would make if its convexity 

 were continued. 



When the rays of the sun fall directly upon a 

 glass DE, (see fig. 52) equally convex on both 

 sides, they will be refracted still more abruptly, 

 and meet sooner in a point or principal focus at 



f. The distance of this focus is, we are informed 

 by the same calculation, equal to the semi-dia- 

 meter of the circle, which the convexity of the 

 glass continued would make. Either this glass 

 or the former, as they collect the rays of the sun 

 into a point, will burn at that point, since the 

 whole force of the rays is concentrated there. 

 The broader the glass in these instruments, the 

 greater will be its power, from its collecting a 

 greater number of rays. 



It is to be observed, that they are only parallel 

 rays ? or those which proceed in a direct line to 

 the surface of the glass, that are thus converged 



