240 optics. 



the way. The ray b will go on in the direction 

 m n; the ray c a in the direction op, &c. The ray 

 C, that falls directly upon the middle of the glass, 

 suffers no refraction in passing through it ; but 

 goes on in the same rectilinear direction, as if no 

 glass had been in the way. 



If the glass had been concave only on one side, 

 and the other side quite flat, the rays would have 

 diverged after passing through it, as if they had 

 come from a radiant point at double the distance 

 of C from the glass ; that is, as if the radiant point 

 had been at the distance of a whole diameter of the 

 glass's convexity. 



If rays come more converging to such a glass, 

 than parallel rays diverge after passing through it, 

 they will continue to converge after passing 

 through it ; but will not meet so soon as if no 

 glass had been in the way ; and will incline to- 

 wards the same side to which they would have 

 diverged, if they had come parallel to the glass. 



Since all the rays of the sun which pass through 

 a convex glass are collected together in its focus, 

 the force of all their heat is collected into that 

 part. Hence, we see the reason why a convex 

 glass causes the sun's rays to burn after passing 

 through it. 



Every lens, whether convex or plano-convex, 

 will collect by refraction the rays of the sun dis- 

 persed over its surface into a point, and thus be- 

 come a burning lens. 



As all the rays which fall upon the lens are 

 united in its focus, their effect ought to be so much 

 the more, as the surface of the lens exceeds that 

 of the focus. Thus, if a lens four inches broad 

 collect the sun's rays into a focus at the distance 

 of one foot, the image will not be more than one- 



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