PHYSICAL PRINCIPLES. 



61 



determining the mean distance of the sun from the 

 earth, a distance which serves as the unit of measure- 

 ment for the solar system and the whole stellar universe. 

 A brief account of how this is effected will now be given. 



Sur 

 Bear 



DARK ROOM 



Concave Mirror 



x> Diagonal Mi 



volving 



Eye of Observer 



Fizeau, a French physicist, made the first attempt at 

 measuring the velocity of light in 1849; and later, in 

 1862, in conjunction with Foucault, a more accurate 

 determination was made by means of an apparatus of 

 which the main features are given in the accompanying 

 diagram. A ray of sunlight is made to enter a darkened 

 room by a narrow slit, and falls on a mirror at the 

 further side of the room, which can be made to revolve 

 with great rapidity. From this it is reflected to a con- 

 cave mirror having its centre of curvature at the revolv- 

 ing mirror. The diagonal mirror is transparent glass, 

 through which the ray passes on its way to the revolv- 

 ing mirror, but on coming back a portion of the light is 



