176 Experimental Philosophy. [Lecture 12. 



but when he approached to unite it with his own, 

 a drawn sword was instantly presented to his 

 breast. A nosegay, or a piece of fruit was 

 offered, but when he attempted to seize it, a 

 death's head snapped at him. 



I mentioned that concave mirrors were fre- 

 quently used as burning-glasses, and a curious 

 experiment may be made by means of them, to 

 show that common culinary fire may be reflected 

 in the same manner as the rays of the sun. If 

 two large concave mirrors are placed opposite to 

 each other, as in fig. 65, at almost any distance, 

 and a red-hot charcoal is held in the focus of one 

 at a, and a match, or any combustible matter, in 

 the focus of the other at b, the match, &c. will be 

 presently set on fire by the reflected flame of the 

 charcoal. 



You have seen, I dare say, the distorted figures 

 which are sometimes painted on boards, and ex- 

 hibited in the shop- windows of opticians. They 

 look like a mere splash of a painter's brush ; but 

 when a mirror of a cylindrical or conical form is 

 set in the middle of the board, a beautiful figure 

 is reflected from it. This shows that what ap- 

 pears to be a casual dash of paint on the board 

 is, in fact, a figure drawn with the nicest mathe- 

 matical precision. When the image is to be 

 rectified by a cylindrical mirror, the lines are 

 only extended, and, by the great law of reflection, 

 the rays from the picture are reflected by the 

 mirror less convergent, and the figure is con- 



