2U6 



APPENDIX 



They are said to converge when considered as flowing from 

 X towards C. And to be parallel as flowing from x towards 

 a and h. C i^ the focus of the converging rays, and the 

 imaginary focus of the diverging rays. The lens here being 

 plano-convex, the focus, as is manifest, is at the distance of 

 the diameter of the sphere, of which the convex surface of 

 the lens forms a portion. The distance from the middle of 

 the glass to the focus is called tlie focal distance. 



Fig. 32. The focal distance of a double convex lens is 

 situated at the centre of the sphere, of which the surface of 

 the lens forms a portion ; — of the lens A B, for instance, f 

 is the focus, and the distance from f to the circumference 

 of the circle is the focal distance, which is equal to half the 

 diameter of the sphere. If another double convex lens FG 

 be placed in the rays at the same distance from the focus, it 

 will so refract the rays, that they shall go out of it parallel to 

 one another. It is evident that all the rays except the mid- 

 dle one, cross each other in the focus /; of course the ray 

 D A, which is uppermost in going in, is the lowest in going 

 out, as G c. 



Pig. 33. If the rays ahc, &lc. pass through A B, and C 

 be the centre of concavity, then the ray a, after passing 

 through the glass, will go in the direction Ic I, as if it had 

 come from C, and no glass in the way : the ray b will go on 

 in the direction wM,,and so on. The point C is called the 

 imaginary focus, 



LESSON 33. 



In fig. 27. A B is a concave mirror, C is the centre of 

 concavity. The rays, which proceed from any remote terres* 

 trial object, as D E, will be converged at a little greater dis- 

 tance than half way between the mirror and C, and the 

 image will be inverted with respect to the object, as de. 

 When the object is more remote than the centre of concavi- 

 ty, or C, the image is less than the object, and is between 

 the object and the mirror, as J e between D E and B C. 

 When the object is nearer than C, the image will be more 

 remote and larger than the object, as D E. If the object be 

 in C, the image and object will be equal and coincide. 



