194 Principal Fccus of concave Mirrors. [Book III. 



convex mirror, will neceflarily appear curved or bent, 

 becaufe the different points of the object are not at 

 equal diftances from the furface of the mirror. All 

 thefe effects will be very apparent from infpecting 

 one of thofe fmall glafs globes, lined with the common 

 amalgam for making looking-glaffes, which are fome- 

 tirnes fufpended in old-fafhioncd apartments. In thefe 

 the company feated in the room, or round the table, 

 are reprefented by very minute images, which appear 

 not at a certain diftance behind as in plane looking- 

 glafies, but very near the furface of the mirror, and al- 

 ways in fome degree curved or diftorted. 



The effects and phenomena of CONCAVE MIRRORS 

 will obvioufly, from what has been faid, be the direct 

 contrary to thofe of the convex kind. The furface of 

 concave mirrors is generally fpherical (or in the form 

 of a globe) though that is not always the moft conve- 

 nient form for optical purpofes, but it is that which is 

 lead difficult to the workmen. 



The general effect of concave mirrors is, we have 

 already feen,-to render the rays more convergent. The 

 point in which the converged rays unite is called the 

 focus of converging rays, but this focus cannot be the 

 fame for all the rays incident on a concave furface. 

 The parallel rays a I, de (fig. 5.) are converged by 

 the mirror at the point F, which is diftant from the 

 mirror one-fourth part of the diameter of that circle, 

 of "which the mirror is a part or fection ; and this is 

 the point which is called the focus cf parallel rays, and it 

 is the real or principal focus of the mirror. The con- 

 verging rays/, h /, are reflected upon the fame prin- 

 ciples more convergent, and unite at the point K, 

 nearer to the furface of the mirror than the principal 

 &cus. In fine, the divergent rays R m and R o, which 

 proceed from the point R, beyond the principal focus, 



unite 



