Pleasant Ways in Science. 271 



By comparing tlie note yielded by vibrations whose velocity 

 was known with that of the buzzing of a gnat, it has been 

 estimated that this little creature vibrates its wings fifteen 

 thousand times in a second.* 



We have spoken of the velocity with which the forms of 

 light waves travel, so that they reach us from the sun in eight 

 minutes, thirteen seconds, and three-tenths of a second. Tre- 

 mendous as is this speed, it looks small when compared with 

 that of the vibrations which the ether particles experience. 

 Thus, " when the ether makes 450 billions of oscillations in a 

 second, the sensation of red light is produced, while 780 bil- 

 lions of oscillations in a second produce violet light.^f 



If light only takes a trifle more than eight minutes to come 

 nearly ninety-two millions of miles from the sun, the time 

 occupied by its passage across an ordinary room would seem 

 too small for possible appreciation, and yet M. Foucault 

 experimentally determined its velocity by operating in such a 

 limited space. His proceedings illustrate the important results 

 that may flow from the employment of accurate means of 

 measuring very small quantities of motion. Before attempting 

 to explain the use made by M. Foucault of Mr. Wheatstone's 

 revolving mirror, let us call attention to a well-known electrical 

 experiment, in which a number of spokes set in a circle are 

 made to revolve rapidly in a dark room. They are then illu- 

 minated by an electric spark, and found to appear at rest. 

 The light has come and gone so fast that the spokes have 

 not had time to make any appreciable change of position. 

 We need not be surprised at this when Wheatstone found 

 that the spark light J (C does not last the millionth part of a 

 second of time," yet this minute time sufficed to make the 

 light vibrations to excite the optical apparatus of the human eye, 

 by communicating to it a quantity of motion sufficient to cause 

 the sensation of light. 



As a step towards understanding Mr. Wheatstone' s 

 measuring apparatus, let the reader take a small looking-glass 

 in both hands, holding it up by the middle of the frame, and 

 gently spin it round so that the bottom shall be where the top 

 was, and vice versa. Let a candle be placed in front of this 

 mirror, so that at the moment it stands upright it shall throw 

 a reflection of it upon the wall. The reflected image will then 

 occupy a certain spot on the wall, and as often as the mirror 

 comes round to the same place, it will throw the reflection on 

 the same spot. If, however, immediately after one reflection 

 has been thrown on the wall, the candle is moved before the 



* Ganot's Physics, by Atkinson, p. 164. 

 f Ganofs Physics, by Atkinson, p. 404. J Noad's Electricity, p. 116. 



