Description of a Burning Murror. 135 
may be easily directed to any given poiat. It is kept in this 
position by two screws. The glass, without changing its 
direction, is moveable on its axis between two collars, and 
may be maintained in al] its positions around this axis“ by 
another screw ; it projects outwards from the mirror, which 
it carries round along with it on its axis, and which, inde-. 
pendently of this motion, may tarn around a particular axis 
perpendiular to that of the object glass. We turn the glass 
on its axis until the particular axis of the mirror as perpendi- 
cular to the plane formed by the incident and reflected rays, 
and we keep it in this position by the screw. Finally, we 
turn the mirror on its particular axis ntil the rays reflected 
are parallel to the axis of the glass, and we are then certain, 
that the image of the sua is upon the object towards which 
the glass is directed. 
The two motions just mentioned are executed one after 
another, and are susceptible of very great precision. With 
respect to the first, when the particular axis of the mirror 
18 perpendicular ta the plane of the incident and reflected 
rays, the edge of the frame which is perpendicular to the 
particular axis of the mirror has a shade which is in a plane 
parallel to that of the incident and reflected rays, and con- 
sequently parallel to the axis of the glass. Thus this shade 
ought to cut the face of a salient index outside of the glass, 
im a straight line which is at the same distance from the. 
axis of the glass with the edge of the frame. This straight 
line being therefore traced on the face of the index, in 
order to execute the first motion, it ig sufficient to turn the 
glass on its axis until the shadow of the frame of the mir+ 
ner coincides with the straight line traced on the index 5 
which is of sufficient precision. 
With respect to the second motion, itis clear that when — 
the mirror is placed in such a way that the reflected ravs 
are parallel to the axis of the glass, if upon the particular 
axis of the mirror, and quite close to the edges of the frame, 
we have rubbed off the plating of the glass on a smali 
piece, this defect in.the plating will produce a shadow which 
will fall on the middle of the straight line of the index, 
Therefore this middle point being marked before hand on 
the index, in order to execute the second motion, it is suf- 
ficient to turn the mirror on its particular axis until the 
shadow of the scratched part falls on this point; which is 
equally precise with the first motion. 
We see, therefore, that assistants im any number may 
each direct the image they severally produce on the point 
indicated for the focus, without being obliged to observe his 
geighbours, It may be noticed, also, that the motion of the 
14 sun 
