MIR 
product of the radius into tlie distance of 
the object, divided by the difference of ra- 
dius and twice the distance of the object, 
will give the focal distance. ' And here we 
are to observe, that, as twice the distance 
of the object is lesser or greater than the 
radius, so the focus will be positive or ne- 
gative, that is, behind the glass or be- 
fore it. 
The image of the object is formed in the 
focus proper to its distance, and, since the 
wiiters on optics demonstrate that the an- 
gles under which the object and its image 
are seen from the centre or vertex of tlie 
mirrour are always equal, it follows, that 
the image will be always in proportion to 
the object, as the focal distance to the ob- 
ject's distance. The position of the object 
will be always erect at a positive focus, or 
behind the speculum diminished by a con- 
vex, and magnified by a concave one. 
Hence^ since a convex has but one, viz. an 
affirmative focus ; so it can never mag- 
nify any object, howsoever posited before 
it. 
The position of the image in a negative 
focus, or that before the glass, will be ever 
inverted ; and, if nearer the vertex than the 
centre, it will be less ; if further from it, it 
will be greater than the object ; but in the 
centre it will be equal to the object, and 
seem to touch it. 
The image formed by a plane speculum 
is erect, large as the life, at the same appa- 
rent. distance behind the glass as the object 
is before it, and on the same side of the 
glass with the object. Those properties 
render this sort of mirrour of most common 
use, viz. as a looking-glass. 
If the rays fall directly, or nearly so, on 
a plane mirrour, and the object be opaque, 
there will be but one single image formed, 
or at least be visible, and that by the se- 
cond surface of the speculum, and not by 
the first, through vyhich the rays do most of 
them pass. 
But if the object be luminous, and the 
rays fall very obliquely on the speculum, 
there will be more than one image formed 
to an eye placed in a proper position to 
view' them. The first image being formed 
by the first surface, will not be so bright as 
the second, w'hich is formed by the second 
surface. The third, fourth, &c. images are 
produced by several reflections of the rays 
between the two surfaces of the speculum ; 
and, since some light is lost by each reflec- 
tion, the images from the second will ap- 
pear still more faint and obscure to the 
MIS 
eighth, ninth, or tenth, which can scarcely 
be discerned at all. 
Mirroursmay be divided into plane, con- 
cave, convex, cylindrical, conical, paraboli- 
cal, and elliptical. 
The properties of cylindrical mirrour* 
are, 1. The dimensions of objects corres- 
ponding lengthwise to the mirrour are not 
much changed ; but those corresponding 
breadthwise have their figures altered, and 
their dimensions lessened the further from 
the mirrour ; whence arises a very great dis- 
tortion. 2. If tlie plane of the reflection 
cut the cyiindric mirrour through the axis, 
the reflection is performed in the same man- 
ner as in a plane mirrour ; and if parallel to 
the base, the reflection is tlie same as in a 
spherical mirrour ; if it cut it obliquely, the 
reflection is the same as in an elliptic mir- 
rour. Hence, as the plane of reflection ne- 
vei’ passes through the axis of the mirrour, 
except when the eye and objective line are 
in the same plane ; nor parallel to the base, 
except when the radiant point and the eyo 
are at the same height; the reflection is 
therefore usually tlie same as in an elliptic 
one. 3. If a hollow cyiindric mirrour be 
directly opposed to the sun, instead of a fo- 
cus of a point, the rays will be reflected in- 
to a lucid line parallel to its axis, at a dis- 
tance somewhat less than a fourth of its dia- 
meter. Hence arises a method of drawing 
anamorphoses, that is, wild deformed figures 
on a plane, which appear well proportioned 
when viewed in a cyiindric mirrour. 
Ill an elliptic mirrour, if a ray strike on it 
from one of its focuses, it is reflected into 
the other. Parabolic mirroiirs, as all the 
rays they reflect meet in one point, make 
the best burning-glasses. 
MISCELLANEjE, in botany, the name 
of the fifty-fourth order in Linnaeus’s “ Frag- 
ments of a Natural Method,” consisting of 
plants, which not being connected together 
by numerous relations, in their habit and 
structure, as the natural families, are assem- 
bled into one head, under the general title 
of miscellaneous plants. 
MISCHIEF. Malicious mischief is an in- 
jury of such a gross nature, to personal pro- 
perty; that although it is not done with a 
felonious intention, or an intent to steal, the 
law has inflicted punishment upon it by 
various statutes. Of tliese are, statute 22 
Henry VH. c. 11 . against destroying dikes 
and bridges in the fens of Norfolk, &c. 
Statute 43 Elizabeth, c. 13. setting lire to 
stacks of corn, &c. and imprisoning persons 
on the borders for the purpose of obtainmi; 
Kk s 
