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 pbincipal focijs 
of the reflector. 
If the object L m be supposed to be moved continually towards 
the centre o, its image l 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 centr . so that if the object were m l the 
image would be l m. 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 m 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 
radiated from the cor¬ 
responding points of 
an image, l 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 
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