May 29, io:'4j 



NA TURE 



Imagine a weight supported by a piece of thread ; the moment 

 that thread is cut the weight falls in a straight line to the ground. 

 If it be desired, therefore, to receive the falling weight in a tube 

 at rest under the weight, and to so receive it that it shall not 

 touch the sides of the tube as it passes through, the tube must 

 be held in an upright position. Take another step, and suppose 

 n™ that it is a question of causing the weight to fall through the 

 tube whilst the tube itself is travelling at a certain rate, say at 



from an astronomical point of view. Consider Fig. 37 for 

 a moment. Here ab represents the path of anything falling, 

 and ocb the angle of the tube destined to receive it. It may be 

 called the angle of slant, but the point is not that we give it any par- 

 ticular name, but that its relation to the velocity of fall is a very 

 fixed and definite one. Accept it as such, and then connect it, 



the velocity of the falling weight. It is perfectly obvious that 

 this cannot be done by holding the tube in a perpendicular 

 position, the tube must be inclined, and the argle of its inclina- 

 tion will vary with the varying relative velocities of tube and 

 weight. The more quickly the weight falls the less inclined 

 must the tube be to receive it. This not only supplies the 

 explanation of the slant of the rain on the windows of the rail- 

 way carriage, but it explains what is very much more important 



not with the falling weight or with the slant of the rain, but with 

 the velocity of the light coming to the earth from any star in tht 

 heavens, and the velocity of the earth in its orbit round the sun. 

 It may be said that tv\ o assumptions are here made, first that 

 light has a velocity, and secondly that the earth does move round 

 the sun. Consider, then, the first of these, the question of the 

 velocity of light. In our day, with all the experimental methods 



and niceties which the labours of those who have gene before 

 have placed at our disposal, this question of the velocity of light 

 can be answered by what may be called a laboratory experi- 

 ment. The first real attempt to answer the question was made 

 some years ago by a Frenchman, M. Fizeau. His method of 

 observation was a beautifully simple one, and has turned out to 

 be highly satisfactory in its results. All the essential parts 

 of his apparatus are shown in Fig. 38. light from a lamp 

 was made to pass through a system of lenses aid was brought to 

 a focus afier reflection from the front surface of a piece of plain 

 glass. The light was then grasped by an object-.; lass and sent 

 out in a parallel beam to a station distant about five miles. 

 There it fell on another object-glass, which again brought it to a 

 focus on a mirror at the end of this second telescope. Then 

 having got the light to the second mirror, it was reflected on its 

 path back again. When the reflected light returned, part of it 

 was allowed to go through the plain glass mirror to the eyepiece 

 seen at the end of the telescope in Fig. 38. At the point where 

 the rays crossed in the first telescope there was interposed the 

 edge of a cogged wheel, to which a great velocity of rotation 

 could be imparted by clockwork, and through the intervals 

 between the teeth of which the light had to pass. Suppose first 



:nnini.jg the ve^city of light. 



that the wheel is at rest. The lamp is lighted, and looking 

 thr.rtigh the cogs of the wheel the observer sees the image of the 

 pmp reflected Lack to him as a star of light from that distant 



mirror by means of the arrangement to which reference has 

 been made. 



