62 THE WONDERFUL CENTURY. CHAP. vm. 



reflected at right angles to the eye of the observer. This 

 involves much loss of light, and in more recent experi- 

 ments the revolving mirror is slightly tilted, so that the 

 returning ray passes beneath the outgoing ray, and is 

 then reflected by a mirror or total reflexion prism to the 

 eye of the observer. Now let us suppose the revolving 

 mirror to be at rest. The various mirrors are first accu- 

 rately adjusted, so that the narrow slit of light (or a fine 

 wire in its centre) is so reflected by the three mirrors that 

 it can be seen in the observing eye piece, and its posi- 

 tion on a fine micrometer exactly noted. If now the 

 mirror is slowly revolved, the line of light will appear 

 and disappear at each revolution; but if it is made to re- 

 volve more than thirty times a second, the line of light 

 will be seen motionless, on the same principle that a 

 rapidly moving luminous object is seen as an illuminated 

 riband. But if light requires any time, however mi- 

 nute, to travel from the revolving to the concave mirror 

 and back again, the mirror will during that time have 

 turned a little on its axis, and the returning ray of light 

 will be reflected to a slightly different point on the 

 diagonal mirror and on the micrometer scale of the eye 

 piece. In Foucault's experiment the distance between 

 the concave and revolving mirrors was only thirteen and 

 a half feet, and he had to make the mirror revolve six 

 hundred times in a second before the returning ray was 

 shifted rather less than one hundredth of an inch. By 

 increasing the speed to eight hundred revolutions the dis- 

 tance was increased to about twelve thousandths of an 

 inch, which, under a powerful magnifier, could be meas- 

 ured with great precision. Having measured with great 

 accuracy the distance between the mirrors, and knowing 



