BURNING GLASS OF ARCHIMEDES. 379 



motion of the luminary. The difficulty would not be very 

 great for three or four persons to do this at the same time,, 

 But if fifty, a hundred, or two hundred persons were em- 

 ployed to form a burning focus in this manner, as none of 

 them could distinguish the image he sent from that sent by 

 another, if one of the images alone should deyiate from the 

 focus, each of the cooperators would try whether it weye 

 his, and hence would arise an agitation and disorder, that 

 would prevent the focus from being formed. This incon- 

 Tenience Mr. Peyrard purposes to remove in a very ingeni- 

 ous way, by furnishing each of his mirrors with an appa, 

 ratus not very complex, which we shall proceed to de- 

 scribe. 



A small telescope supported on a stand, and furnished Contrivance for 

 with two wires crossing each other in the focus of the "i^J^^^'"^ -^ 

 glasses, may easily be directed to ihe point, to which the 

 image is to be conveyed. In this direction it is fixed by two 

 screws. This telescope, without changing its direction, is 

 movable on its axis between two collars, and can be kept 

 in any position round this axis by another screw. On this 

 telescope is fixed the mirror, which it carries with it when it 

 turns round its axis; and Avhich, independent of this mo- 

 tion, is capable of turning round another axis, perpendi- 

 cular to that of the telescope. The telescope is to be turned 

 on its axis, till the axis of the mirror is perpendicular to the 

 plane formed by the incident and reflected rays, and in this 

 position it is to be fixed by a screw. Lastly the mirror is 

 to be turned on its axis, till the reflected rays are parallel to 

 the axis of the telescope ; and then the image of the sun 

 must strike the object at which the telescope points. 



The two movements here mentioned are executed one af- First more- 

 ter the other, and are capable of considerable precision. ™®"*' 

 With respect to the first^ when the axis of the mirror is per- 

 pendicular to the plane of the incident and reflected rays,- 

 the edge of the frame, which is perpendicular to the axis of 

 the mirror, throws its shadow in a plane parallel to the in- 

 cident and reflected rays, and consequently parallel to the 

 axis of the telescope. This shadow therefore, or the boun- 

 dary of the light reflected from the mirror, will cut an index 

 projecting from the telescope in a right line at the same dis- 

 tance 



