THE VELOCITY OF LIGHT. 215 



must incline your telescope forwards to catch the ray of 

 light, that is to make it pass along the same path in the 

 telescope. The amount that the telescope has to be moved 

 is the angle s E s'. This explanation is quite clear on the 

 corpuscular theory, but the same explanation has been 

 shown by Stokes to apply to the undulatory theory also. 



Bradley called this source of error the aberration of light, 

 and found that it amounted to 20". The tangent of this 



angle, or = hence, from Bradley's observations, 



A E 1U^ 1 U 



the velocity of light is 10,210 times the velocity of the 

 earth. Now the distance of the earth from the sun is 

 about 92,000,000 miles, hence the circumference of the 

 earth's orbit is this quantity multiplied by 6 '283 1 8 ; and 

 this distance is accomplished in 365J days. This gives us 

 19 miles a second as the velocity of the earth, and 194,000 

 miles a second as the velocity of light. 



A more accurate value has since been obtained by 

 Wilhelm Struve, and since confirmed, of 20", 445. This gives 

 us a proportionally altered value for the velocity of light. 



The results which had now been obtained, while they 

 were sufficient to give a fair idea of the amount of the 

 velocity of light, still left room for much more important 

 work. In both the methods hitherto spoken of, the velocity 

 of light was calculated from an assumed knowledge of the 

 distance of the sun from the earth. But you will easily 

 see that this is a very difficult thing to determine, when 

 you remember the expense and trouble that was spent in 

 1874 to find it more accurately by the method of the transit 

 of Venus. It was an important object then to measure 

 the velocity of light by some independent means. This 

 difficult problem was first attacked by M. Fizeau ; and with 

 perfect success. 



We have before us the apparatus employed in this 

 splendid investigation. The problem seems hard enough. 

 A beam of light was to be sent from the observer's station 

 to a distant reflector, and the time occupied before it 

 returned was to be measured. From the figures I have 

 already given you, you will see that this must be a very 

 small fraction of a second. The method employed by M. 

 Fizeau was extremely ingenious. Here is a disc that is 

 blackened to prevent it from reflecting much light, it has 



