THE FLAME-MANOMETER. 397 



causes often headache. It answers better to use an 

 opera-glass, moving it from side to side before the eyes, 

 while the head is kept in a steady position. 



The best mode of observing the flame is by means 

 of a moving mirror. A square box, the sides of which 

 are made of looking-glass, made to rotate in the vicinity 

 of the flame with moderate velocity, enables a great 

 number of persons to observe the flame very con- 

 veniently. 



The image of an object before a mirror is always seen 



FIG. 235 (i real size). 



as far behind the mirror as the object is before the 

 mirror, and it appears upon the straight line which is 

 drawn from the object perpendicular to the surface of 

 the mirror (see further on, Art. 39). In fig. 235, let 

 1 1 s p represent the box, with its sides of looking-glass 

 as seen from above at any instant; and let II II, III 

 III, IV IV, V Y, represent the changed positions 

 which the side 1 1 occupies in successive instants of 

 time, while the box is rotating; let/ be the flame. If 



