440 CONCAVE MIRRORS. 



Whenever, as in the preceding case, reflected ra\ s 

 converge, and coincide at a point in front of the mirror, 

 on the same side as the object, the image formed is 

 called a real image, for it can be received on a screen. 

 But if the image has no real existence, and the luminous 

 rays do not actually meet after reflection, but their 

 prolongations coincide in some point, as is the case 

 with images produced by plane mirrors, the appearance 

 is called a virtual image. The distinction may be ex- 

 pressed by saying that real images are formed by the 

 reflected rays themselves, and virtual images are formed 

 by the prolongations of the incident rays. 



The point B situated between A and the centre of 

 curvature c gives an image at 6, between the image a 

 and the centre. A body placed above the centre of 

 curvature will present a series of points to the mirror, 

 and a corresponding series of images of points will give 

 by their combination an image of the body, but inverted ; 

 that is, the whole image of the body will appear upside 

 down, but of the same size as the body itself. This 

 may be clearly seen from the relative positions of J., B, 

 and a, b. If the body were placed to the right of c, a 

 corresponding inverted image would be formed to the 

 left of c. 



Concave mirrors made of glass with silvering at the back produce 

 multiple images, which with mirrors of this kind have a still more 

 disturbing effect than in the case of plane mirrors. Proper images 

 are only obtained if the front of the curved mirror is formed by a 

 polished metallic surface. Carefully ground metallic mirrors of this 

 kind, such as used for telescopes, are expensive, because they are 

 difficult to construct ; but concave mirrors, sufficiently accurate for 

 studying the images produced, may be prepared by the student, 

 using for the purpose an alloy of lead and tin. The mirrors thus 

 produced have a somewhat wavy surface, and give slightly blurred 



