Description of a Burning Mirror. 137 
plate bas’a stalk which traverses a square hole made in the 
fixed plate. The square plate may be moved from right to 
left, and lowered or raised : a screw placed behind the fixed 
plate keeps the moveable plate in the position which we 
wish to give it. : 
The moveable plate ought to be placed in such a way that 
the straight line XX if prolonged passes by the axis of 
tke object glass, and is parallel to the particular axis of the 
mirror, and so as that the distance from the straight line 
YY to the axis of the object glass is equal to the distance 
from the straight line IK to this same axis. The plate ZZ 
being so placed,’ it is evident that the straight line YY will 
be parallel to IK, and that the straight line drawn from the 
point at which the axis of the mirror cuts IK to the point 
at which XX cuts YY, will be parallel to the axis of the 
object glass. 
The piece OO’ is_a spring fixed at O’ with the square. 
This spring is traversed at Q by the screw RQ. On turn- 
ing this screw, the extremity of the square presses the 
pivot OO on the frame of the glass. 
' The square HHH is surmounted by an assemblage of 
pieces represented in fiy. 2. The piece a b and the pivot 
~. OO are ranged in an invariable manner. The extremity cf 
the square and the piece VV have a square hole which re- 
ceives the pivot OO. When we turn the screw T, the 
piece ab may be moved before or behind; and when we 
turn the screw S, the piece VV may be moved from right 
to left with the piece a 0. 
In order to give the axis of the mirror a position perpen- 
dicular on the axis of the object glass, and to place the 
moveable plate ZZ (fig. 1.) m such a manner that the 
straight line drawn from the point at which the axis of the . 
mirror cuts the line IK, to the point at which XX cuts YY, 
may be parallel to IK, and finally, in order to place the 
straight line YY paraliel to IK, I act in the following 
manner : ’ 
I place the mirpor in such a manner that the straicht 
Jine IK cuts at right angles the axis of the object glass. I 
turn the screw T, and bring the loweredge of the frame to 
be tangent to the circular surface M’M’, which is parallel to 
the axis of the object glass. I afterwards turn the screw T 
in order to fix the piece a (fig. 2.) in an invariable manner. 
I afterwards direct the axis of the mirror on a point of a 
plain surface placed ata certain distance. This point must 
be in the vertical plane which passes by the eye of the ob- 
‘server and by the centre of the sun, and this plane must be 
perpendicular 
