Description of a Burning Mirror. 139 
brought upon the same object, and that they will remain 
fixed upon it as long as we please, 
I have said that it will require a3 many persons as there 
are mirrors ; but it is easy to foresee that a single person 
might easily direct ten or even twenty mirrors, without 
having reason to apprehend any displacement of the focus, 
or the dispersion of the images. 
If the object on which we wish to bring the images of the, 
sun was in motion, it would be necessary that each mirror 
should be directed by two persons: one would be instructed 
constantly to direct the axis of the object glass on the 
thing in motion, while the other would be instructed to 
throw the shadow of the straight line TK on the straight 
line YY, and the shadow of the pivot NN on the straight 
line XX, so as that this straight line might divide the 
shadow of the pivot into two equal parts. 
Such is the burning mirror which I have contrived. 
The construction is very simple ; the method of using it 
is easy ; and it is beyond doubt, that by its means we may 
reflect and fix on an object at rest, or in motion, the solar 
rays, in-as great a quantity as we please. 
I shall now show what are the effects which my mirror 
is capable of producing. 
Buffon ascertained by several experiments, that the light 
of the sun reflected by a looking glass. did not lose, at 
short distances, more than one half “by reflection; that it 
lost, at great distances, scarcely any of its force from the 
thickness of the air hicks it had to pass through ; and that 
its force was diminished solely in an inverse ratio to the 
augmentation of the surfaces which it might occupy upon 
planes perpendicular on the reflected rays * 
This being granted, let us suppose that the glasses of 
every mirror are each of them five decimetres high and six 
broad. I take them to be greater in breadth than height, 
in order that the images reflected may have their height 
nearly equal to their breadth; for the ravs of the sun being 
always perpendicular on the. axis of every glass, while they 
are more or less inclined on the line IK, if the height of 
the glasses was equal to their breadth, "when the rays of 
the sun were not perpendicular on the plane ot the glasses, 
the heights of the tages of the sun would be always smaller 
thau their breadths. 
In order to calculate with more facility the effects of my 
mirror, | suppose that the glasses are of a circular form, 
* See the Supplement to Buffon's Natural History, 4to edit. Paris 1774, 
tome i, p. 401 and #05, ° 
having 
