408 THE OUTGO OF ENERGY 



the distance between the luminous point and the 

 . mirror. 



As the distance between the luminous point 

 and the mirror increases, the distance between 

 the mirror and the image diminishes. 



Move the lantern towards the optical box, and 

 thus bring the luminous point towards the mirror. 



As the distance between the mirror and one 

 conjugate focus diminishes, the distance between 

 the mirror and the other conjugate focus in- 

 creases. As one focus approaches the mirror, the 

 other recedes. 



3. Place the mirror 5 cm. from the luminous 

 point. The luminous point is now at the centre 

 of the sphere of which the reflecting surface is 

 a segment. The incident rays are therefore all 

 radial, i. e. perpendicular, to this surface. Con- 

 sequently all the rays will be reflected to the 

 point of origin. The incident and the reflected 

 rays will coincide. The centre of the reflecting 

 surface and its optical image will also coincide. 

 The conjugate foci coincide. 



Virtual Image. 1. Place the mirror at a 

 distance from the luminous point less than the 

 principal focal distance. 



The reflected rays will diverge. They will 

 appear to proceed from a point lying behind the 

 mirror. The distance between this unreal or 



