OPTICAL IMAGES. 



centre o, a position will be found at which the image will 

 be distinct. The card in this case should be so small as not 

 to intercept too much of the light radiated from the candle to the 

 mirror. 



12. If the candle be now supposed to be gradually removed to 

 greater and greater distances from the reflector, the image will 

 approach nearer and nearer to the middle point F of the radius 

 o B, and when its distance attains a certain limit, the image will 

 be formed at F. However much the distance may be further 

 augmented, the image will remain stationary at F. 



This point F being therefore the place at which the images of 

 all very distant objects are formed, is called the PRINCIPAL FOCUS 

 of the reflector. 



If the object L M be supposed to be moved continually towards 

 the centre o, its image I m will also move towards o. When the 

 object is moved past the point o towards the reflector, its image 

 will be found outside the centre, so that if the object were m I the 

 image would be L 3r. In passing the centre o, therefore, the 

 object and image interchange places. 



So long as the object is outside the centre, it will be greater 

 than its image, but when inside the centre it will be less. The 

 reflector, therefore, acts as a magnifier, or the contrary, according 

 as the object is between o and F, or outside the centre o. 



All these effects can be verified experimentally by receiving the 

 image on a card in the manner described above. It is evident 

 that in all these cases the images are real. 



If the object L 31 be placed between F and B, as in fig. 7, the 



pencils of rays which 

 diverge from the seve- 

 ral points of the object 

 will be reflected, di- 

 verging as if they had 

 " o radiated from the cor- 

 responding points of 

 an image, I m, at a 

 certain distance be- 

 hind the reflector. This image will be similar in position with the 

 object, that is erect, and it will be greater than the object in its 

 linear dimensions, in proportion to its distance from the centre o 

 of the reflector. 



Since the image in this case is behind the reflector it will be 

 imaginary. 



If the object be moved towards B, the image will also move 

 towards B, and if the object be moved towards F, the image will 

 move from B, and will recede through spaces much greater than 

 88 



