OPTICS. 115 
parallel central rays and the mirror, the rays will be reflected diverging, as 
if they came from a point behind the mirror (fig. 18). 
All that has just been said applies equally to rays reflected from points 
not in the axis, as an imaginary axis may be drawn through the centre of 
curvature and the radiant, provided the mirror be sufficiently large. 
On the principles just enumerated, it becomes easy to determine the 
nature of images formed in concave mirrors. If an object, AB (fig. 19), 
be placed between the centre, C, and the focus F’, the mirror exhibits an 
image, ab, inverted and magnified, and situated at a greater distance from 
the mirror. For an object at ab the image will be inverted, diminished, 
and nearer to the mirror. The further the object from the mirror, the 
nearer is the image to the focus of parallel rays; if this distance becomes 
infinite, as in the case of the sun or stars, the image will be in the focus. 
An object at the centre of curvature will have an image there also, and 
inverted. Objects at the focus. and between this and the mirror, will 
exhibit no image whatever in front. 
The images formed in concave are very different from those of plane 
mirrors. ‘The latter appear as if proceeding from a point behind the 
mirror, thus diverging, while the former converge. The images formed by 
a concave mirror may be thrown ona screen of white paper or ground 
glass. 
The radius of curvature of a concave mirror may be readily determined 
by observing the place before the mirror at which the image of the sun is 
formed on a screen. This image will, of course, be in the focus of 
parallel rays, and twice the distance thus formed will be the radius of 
curvature. 
Although no image is formed in front of a concave mirror by objects 
placed between the focus and the mirror, yet an apparent image will be 
formed behind it. If in fig. 20 AB be the object, the normal ray, An, will 
be reflected back in the direction nAC. Ae, however, which is parallel to 
the axis, will be reflected to F'; nAC and eF’, produced backwards, will 
intersect at a, where will be the image of A. Obtain b, the image of 
the other extremity, B, of the object, and ab will be the image required. 
It will be observed that this is larger than the object, lies behind the mirror. 
and is erect. 
A spherical convex mirror (fig. 21) has no actual focus, as the reflected 
rays do not unite ; they diverge, however, after reflection, as if they came 
from one and the same point behind the mirror. When the rays are 
parallel to the axis, this point, v, will be half way between the centre of 
curvature and the mirror, thus corresponding to the focus of parallel rays 
in the concave mirror. The focus of parallel rays in the convex mirror is 
called the virtual or apparent, to distinguish it from the real or actual 
focus of the concave mirror. A convex mirror exhibits a direct, but 
diminished image, ab, behind the mirror (fig. 22), of which we may become 
easily convinced, by comparing the explanation of fig. 20, and considering 
F as the focus. | 
When the rays proceeding from a luminous point, and reflected from a 
ICONOGRAPHIC ENCYCLOPZDIA.—VOL. I. 19 289 
