Convex burning Qaffes. [Book III. 



refracted from the perpendiculars Ca, C, acd arc 

 confequently divergent. 



Converging rays, if their point of convergence is pre- 

 cifely a,t C, the center of the concavity e D /, will not 

 fuffer any refraction, becaufe they are perpendiculars, 

 as already explained, therefore have no obliquity of in- 

 cidence. If, on the other hand, the rays tend to a point,' 

 fiich as n, nearer to the furface than the center of the 

 concavity C, then they are rendered more convergent, 

 for the rays q <?, r i, which naturally tend to that point, 

 are refracted from the perpendiculars C e, C /', and con- 

 verge at o, nearer the concave furface. 



Laftly, if the converging rays tend to a point /, which 

 is beyond the center C, they are rendered lefs conver- 

 gent. For the rays se, //', which would naturally 

 unite at that point, are refracted from the perpendicu- 

 lars Ct, d, and unite at k 3 which is more diftant 

 dill. 



Diverging rays in the fame circumftances are ren- 

 dered more divergent. For the rays E <?, E./, diverg- 

 ing from the point E, inftead of proceeding towards 

 and x y are refracted from the perpendiculars, and are 

 directed towards y and z. 



From the property which all fpherical convex fur- 

 faces have of rendering parallel rays pafiing out of a 

 rarer medium convergent, glafles made in this form 

 are very commonly ufed as burning glafies; and as the 

 fun's rays, proceeding from fo vail a diftance, may be 

 confidered as parallel, the focus of parallel rays will of 

 courfe be their burning point. 



A LENS is a tranfparent body of a different denfity 

 from the furrounding medium, and terminated by two 

 furfaces, either botl* fpherical, or the one plane and 

 tlie other fpherical, whether convex or concave. They 

 are therefore generally diftinguifhed by their forms, 



and 



