Description of a Burning Mirror. 145 
Tustead of employing glasses which should be two metres 
in height, we might employ four glasses of a metre in 
height, which we might arrange in the same way, and the ' 
effect would be the same. 
Betore concluding, | shall say something of the burning 
mirrors which have been contrived to produce effects at 
great distances. Buffon’s mirror was the last I know of. 
‘This instrument is.composed of 168 plain glasses mounted 
in iron frames. These glasses, which are six inches high 
by eight broad, are moveable in every direction. 
The above mirror has two prominent defects. It requires 
about half an hour to adjust it, 2. e. to bring to the same 
point the 168 images of the sun reflected by the glasses. 
But the glasses being adjusted by each other, and the images 
reflected removing every moment from their first positions, 
it is evident, that when the operation is concluded the 
amages must necessarily have removed from the focus. 
‘Hence it follows, that at every second the focus is displaced 
or enlarged, and loses its activity. 
Let us suppose, for a moment, that, the mirror being ad- 
justed, the images of the sun are exactly applied upon 
each other: I assert, that in this case M. Buffon’s mirror 
has all the properties, aud nothing but the properties, of a 
‘parabolic mirror composed of plain glasses. 
Let us suppose in fact a certain number of plain glasses 
BC, DE, &c. (fig. 4,) placed as we please, provided their 
‘centres GH, &c. reflect the solar rays IG, KH in a point 
F. By the point F draw the straight line AL parallel to. 
the solar rays IG, KH; on this parallel take a point A on 
the prolongation of LF, and describe a parabola MAN, the 
origin of whose axis is the point A, and the focus is the 
point F. 
If this parabola makes a revolution around its axis, it. 
will describe the surface of a parabolic conoid. Let us 
now suppcese that the glasses BC, DE, &c. approach to or 
remove from the point F by moving parallel to themselves, 
following the straight lines GF, HF, until they are tangent 
tothe conoid. Itis evident that the points of contact will 
be the centres of the glasses, and that the centres of these 
T° placed at J c, d ¢ will reflect the solar rays OH, PG, 
‘&c. at the point F, in the same manner as they would re- 
flect the solar rays IG, KH, &c. when these glasses were 
placed at BC, DE, &c. I conclude therefore, that if, Buf- 
fon’s mirrors being adjusted, the images were exactly ap- 
‘plied upon each other, this mirror would have all the pro- 
‘perties, and no more than the properties, of a paraboli¢ mir- 
© Vol. 37. No. 154, Feb. 181). K ror 
