62 The Velocity of Light. 



finite velocity, and which also gives a rational explanation of Roe- 

 mer's discoveries. 



We have now seen the ways in which the velocity of light was 

 discovered by astromonical calculations, and it only remains for us 

 to see how these results were corroboi'ated by actual experiments. 

 This was first performed by M. Fizeau. The apparatus with which 

 he determined the velocity of light is in principle very simple, and 

 consists of a modification of the plan we at first mentioned.* M. 

 Fizeau deduced from an average of twenty-eight experiments that 

 the velocity of light was 315,000 kilometers a second. 



The last and most important experiment on the velocity of light 

 was made by Foucault, His apparatus in theory was not better than 

 Fizeau's, but the patience and determination with which he carried 

 out his experiments have procured a more correct result, and a re- 

 sult which must now be universally accepted as the true solution of 

 the problem of the velocity of light. The apparatus which he used 

 was in principle as follows : — A screen with a small slit in it, and in 

 that slit a piece of platinum wire to form an object. Through 

 the slit a ray of light was thrown by a reflector known as the helio- 

 stat ; the light which passed through the slit was collected by a lens 

 and thrown on to a revolving mirror, from which it was reflected, 

 to a concave mirror placed so as to send it back in exactly the 

 same direction on to the revolving mirror. If the revolving mirror 

 was stationary the light would be reflected by it to exactly the point 

 from which it started, and if light traversed space instantaneously, 

 and the mirror revolved at any rate, however great, it would also be 

 reflected back to the same point ; but if, on the contrary, light has 

 a finite velocity, and the mirror were revolving, the light would im- 

 pinge upon the mirror in a slightly changed position, and would, 

 of course, be reflected ofl"in a direction slightly inclined to the line 

 along which it first came to the revolving mirror. If we measure 

 the angle formed by the original direction of the ray, and that of 

 the reflected ray, which can be proved to be double the angle 

 formed by the two positions of the mirror, we shall have found out 

 the size of the segment of a circle or angle which the mirror has 

 revolved, and if we know the number of revolutions the mirror 

 makes per second, and the distance the light has to travel, viz. : 

 twice the distance from the revolving mirror to the fixed mirror, it 

 will be easy to calculate how far light travels in a second. From 

 the average of his experiments, M. Foucault was led to conclude 

 that light travelled at the rate of 298,187 kilometres, or 185288-537 

 miles [1 kilometre :=■ '6213824 miles] a second. This is now 

 generally accepted as the true velocity of light. 



• The apparatus was then described. 



