138 Description of a Burning Muror, 
perpendicular on the plain surface which we have mentioned. 
By this point, I draw a horizontal straight line, and setting 
out from this point, 1 take a second point pelined is as far 
from the first as is the centre of the mirror from the axis 
of the object glass. I unscrew at S.J turn the ubject 
glass on its axis; the mirror also on its particular axis, and 
1 advance or retract the piece VV until the centre of the 
image reflected falls on the second point. TI fix the piece 
VV. I afterwards place the piece ZZ in such a way that 
the shadow of the straight line [IK falls on the straight line 
YY, and that the shadow of MM may be divided into two 
equal paris by thestraight line XX, and I fix the piece ZZ. 
The mirror being thus mounted, it is evident that, what- 
ever be the point on which we may have directed the axis 
of the object glass, the shadow of NN, and consequently 
all the rays reflected by the surface ‘of the mirror, will be 
parallel to the axis of the object elass, provided the shadow 
of IK falls on YY, and the shadow of NN is divided inta 
two equal parts by the straight line XX. 
The mirror bemg thus “arranged, the following is the 
method of using it: 
In order to bring the image of the sun on any given ob- 
ject, we must first direct the axis of the object glass on any 
given point of the object; 2dly,turn the object "glass on its 
axis until the shadow of the Jine IK falls on.the line YY; 
3dly, turn the mirrer on its particular axis until the stualiir 
of the band MM is divided imto two equal parts by the 
straight line XX. 
These three operations being fi finished, it is evident that 
the image of the sun will fall on the given object 5 or, to 
speak more correctly, the centre of the image reflected, in- 
stead of being on the point of the object upon which we 
have directed the axis of the object glass, will be at a di- 
stance from it equal to that which is between the centre of 
the mirror and the axis of the object glass. 
If in prop rtion as the sum advances we take care to 
keep the shadow of jhe straight line 1K on the straigbt line 
YY, and the Shadi of NN on the straight line XX, so 
that the straight line XX may divide the shadow of NN 
into two equal parts, it is evident that the i image will pre- 
serve its first position as long as we please. 
Let us now suppose that we have a great number of 
these mirrors ; that they are placed by the side of one 
another in rows above each other; and Jet us suppose 
that each of these mirrors is directed by a single individual. 
It is evident hat the images reflected by the mirrors will be 
brovghs 
