232 Effect* ofjpberica! Surfaces, [Book lit 



I'm* 1C from the poihts I and M. This point I is 

 called the focus of refracted parallel rays. 



In the, fame manner M is the focus of rays coming 

 out of the denfe medium, and CM : E M :: fine R ; 

 fine I. 



Parallel rays incident on the convex furface of a 

 ' denfer medium, or the concave furface of a rarer me- 

 dium (Fig. 17.) converge after refraction; and on the 

 contrary, if they fall on the convex furface of a rarer^ 

 or the concave furface of a denfer medium (Fig. 18.) 

 they diverge after refraction. The focus of parallel 

 rays thus found is called the principal focus, as for- 

 merly explained. 



Let CL(Plate XVI. Fig. 19.) be the focus of di- 

 verging rays incident on the furface AB of a denfer* 

 medium ; to find the focus after refraction draw QJ: 

 an incident ray, and fnppofe T to be the focus of rays 

 parallel to C Q^ incident on the concave furface A E B, 

 and make C P equal to C T, from I draw I q parallel 

 to C P, and q will be the focus of refracted rays. Let 

 / be the focus of rays parallel to Qj3 incident on the 

 convex furface, and make Cp equal to C/. Then, 

 . fince a ray parallel to C P incident on the concave fur- 

 face would after refraction converge ,to P, a ray di-. 

 verging from P will after, refraction go parallel to C P. 

 Now the courfe of the ray QJ is the fame, whether it 

 is confideied as diverging from Q^or P, therefore the 

 direction of one of the rays diverging from Q^will be 

 in the line QJ q. Suppofe" now the ray q 1 to be turn- 

 ed back, its progrefs ,will be the fame as if it had di- 

 verged from p; but all rays diverging from p, and in- 

 cident on the concave furface, move after refraction 

 parallel to Cp, p being the focus of parallel rays inci- 

 dent on -the other furface, therefore the ray p I muft 

 after refraction move parallel to C/j but its direction 



mull 



