584 MR. WHEATSTONE ON SOME EXPERIMENTS TO MEASURE THE VELOCITY 



the motion of the spark be downwards, the deflection of the line should be as in fig. 2; 

 and if its motion be upwards, it should be deflected as in fig. 3.j 



When the apparatus was made to revolve rapicJly, the sparks passed in the same 

 manner as when it was at rest, and no deviation of the extremities of either of 

 the two sparks from the same vertical line was observed. The apparatus revolved 

 fifty times in a second, and as a difference of the twentieth part of the circumference 

 described by the balls could have easily been observed had it existed, we may safely 

 conclude that the spark passed jointly through the air and the metallic conductor in 

 less time than the thousandth part of a second. 



§2. 



Having failed to observe any deflection of the spark by the means just mentioned, 

 T found it necessary, if 1 would continue the inquiry, to contrive some more effectual 

 means of prosecuting it. It occurred to me that the motion of the reflected image 

 of the electric spark in a plane mirror would answer all the purposes of the mo- 

 tion of the apparatus itself connected with the spark. Several advantages, it was 

 evident, would result from this substitution ; the apparent motion of the reflected 

 image in a small moving mirror would be equal to an extensive motion of the object 

 itself; the same mirror might be presented to any object to be examined, thus 

 forming, with its moving machine, an independent and universally applicable instru- 

 ment ; and many experiments might be tried, which, without this expedient, would be 

 difficult or impossible to perform, from the size or immobility of the apparatus. 



The most convenient form of the revolving mirror is represented in fig. 4 ; it ro- 

 tates on a vertical axis, and in its motion successively assumes every vertical plane. 

 If a luminous point, the flame of a candle for instance, be placed at any distance 

 before this revolving mirror, the successive places of its reflected image will describe 

 a circle, the radius of which is equal to the perpendicular distance between the lumi- 

 nous point and the axis of rotation. The angular velocity of the image is twice that 

 of the mirror ; the entire circle is consequently described while the mirror makes a 

 semi-revolution ; and if the back of the mirror be also a reflecting surface, the image 

 will describe two entire circles during one revolution of the mirror. If the motion 

 exceed a certain rapidity, the successive images leave their impressions on the retina, 

 and the eye, properly placed, takes in the view of a perfectly continuous line of 

 light, being an arc of the circle described, which arc is larger in extent in proportion 

 to the proximity of the eye to the mirror. 



If now, while the mirror is in motion, the luminous point be moved in a direction 

 parallel to the axis of rotation, the composition of the two motions of the image, the 

 one depending on the motion of the object, the other on the motion of the mirror, 

 will give rise to a diagonal resultant ; and if the number of rotations made by the 

 mirror in a given time are known, the direction and velocity of the moving point 

 may be calculated. 



