Chap. ^.] t 227 ] 



CHAP. VL 



<DF THE PRINCIPLES OF DIOPTRICS. 



Dioptrics relate to the Effect; of Refra3ion.In what Manntr to find, 

 the aSual Situation of an Qbjecl feen in a different Medium. Thf 

 apparent Situation of an Qbjefl feen through a Ghifs Window dif- 

 ferent from the real one.~The Effects of tranfparent M:dia <iuitb 

 .fpherical Surfaces on the Rays of Light vbbicb are tranfmitted through 

 them. Rules for finding the Focus of Rajs pajjlng through fucb Me- 

 diums.- Theory of Lenfes. Plano-convex and flano-c onca-Tjs Lenfts. 

 Rules to find the Focus of a Lens. Focus of diverging Rays in- 

 tercepted to a Lens^^Focus of a Sphere, 



TI^E word dioptrics is compounded of two Greek 

 words, which mean to fee through-, and this 

 branch of the fcience of optics furniihes us with rules 

 and principles to determine with accuracy the effects 

 of tranfparent mediums upon the rays of light, what- 

 ever may be the form of the furface through which 

 the light is made to pafs. Thus, in an extenfive fenfe, 

 idioptrics would include the whole theory of optical 

 glafles, and even of vifion itfelf*. For the convenience 

 of the ftudent, however, I have thought it bed to treat 

 ieparately of thefe fubje&s, and {hall therefore in this 

 ch ipter include only the general principles. 



As the whole of dioptrics is founded on the laws of re- 

 fra<tion, it will be necefTary to recur to what was advan- 

 ced on that fubjecl: in our fourth chapter, and to recollect 

 that the angle of refraction is in a given ratio to the angle 

 of incidence. Let H F for inftance be a ray of light 

 incident on the furface AB, (Plate XIV. Fig. 10.) of 

 fc denfe medium ABCD,.fuppofe it to be glafs fur- 

 i rounded 



