MICHEL80\'S KECENT REiSEARClIES ON LIGHT. 451 



While the miuiber of physical sciences has received mimerous addi- 

 tions duriiij;- the hist half-cent my, new affiliations and a more intimate 

 correlation have been manifested. In this mntual lielpfnlness light has 

 played an iniportant part. The optical method of stadyiny sound, and 

 the many varieties of flame apparatus, have made acoustics as intelli- 

 gible through the eye as through the ear. 



Velocity being expressed by space divided by time, it is evident tnat 

 in measuring an immense velocity we must have at our command an 

 enormous distance, such as we find only in astronomy, or else possess 

 the means of measuring fractions of time as small as one-millionth of 

 a second. The tirst successful attempt to measure such a velocity was 

 made by Wheatstone in 1834. Discharges from a Leyden jar were sent 

 through a wire, having two breaks in it one-fourth of a mile apart. The 

 wire was in the form of a loop, so as to bring the breaks into the same 

 vertical line. The sparks seen at these breaks were reflected by a mir- 

 ror at the distance of 10 feet, and revolving eight hundred times per 

 second. The images of the two sparks were relatively displaced in a 

 horizontal direction. As the displacement did not exceed one-half of 

 an inch, the time taken by electricity to go from one break to the other 

 was less thin a millionth of a second. Since the distance was one- 

 quarter of a mile, the electricity travelled hi that case at the rate of 

 288,000 miles a second. If this experiment is interpreted to mean that 

 electricity would go over 288,000 miles of similar wire in one second, as 

 it probably often was at that time, the conclusion is lallacious. The 

 velocity of electricity, unlike that of sound or light, diminishes when 

 the length of wire increases. 



In 1838, Wheatstone suggested a method for measuring the velocity 

 of light, which he thought was adequate for giving not only the abso- 

 lute velocity but the difference of velocity in different media. 



In that year Arago communicated to the French Academy the details 

 of an exi)erimeut which he thought would give the velocity of light in 

 air or a vacuum. As his own health was broken down (he died in 1853) 

 he appealed to two young French physicists to undertake the experi- 

 ment. On July 23, 1849, Fizeau, by a method wholly his own, made a 

 successful experiment. A disk cut at its circumference into 720 teeth and 

 intervals, and made by Breguet, was rapidly rotated by a train of wheels 

 and weights. A concentrated beam of light was sent out through one 

 of the intervals between two teeth of the disk, which was mounted in a 

 house in Suresne, near Paris, and was sent back by a mirror placed on 

 Montmartre, at a distance of about 5 miles. The light, on its leturn, 

 was cut off from tiie eye or entered it, according as it encountered a 

 tooth or an interval of the disk. If the disk turned 12.G times in a sec- 

 ond the light encountered the tooth adjacent to the interval through 

 which the light went out. With twice as many rotations in the disk 

 the light could enter the eye through the adjacent interval. With 

 tUree times the original velocnty, it was cut off by the next tooth but 



