218 LECTUEES TO SCIENCE TEACHERS. 



rate of rotation was measured by a counter which you may 

 see attached here. We know then that light takes the 

 same time to pass over 17,266 metres, as a tooth of the disc 

 takes to reach the position of a notch. But to do this it 

 must turn through the -2 (To o^ n ^ a revolution, therefore 

 this interval of time is a second divided by the product of 

 12*6 and ^fiflw- Hence, in one second light passes over 

 a distance of 17,266 metres multiplied by 25, 200. This is 

 435,000,000m. a second. 



The final result arrived at by Fizeau was that light 

 travels at the rate of 70,948 leagues in a second, there 

 being 25 leagues to the degree. 



The experimental investigation that has now been dis- 

 cussed is one of the most ingenious and difficult that have 

 ever been undertaken. The chief difficulties arise from the 

 want of light in the reflected image, and the illumination 

 of the field by extraneous light. The chief obstacles to 

 an exact measurement by this means are, 1, the difficulty 

 of measuring the velocity of rotation of the notched wheel 

 at any moment. 2. The eclipse of the light is found 

 practically to continue during a considerable variation of 

 the velocity of the disc. Hence it becomes necessary to 

 measure the velocity when the eclipse is first produced ; 

 and also the greater velocity when the eclipse ends. Hence 

 it is necessary that the intensity of the source of light 

 should remain constant. 3. It is a very difficult observa- 

 tion to determine exactly when an eclipse takes place. This 

 depends very much on the amount of sensitiveness of the 

 eye, which varies with the size of the pupil, a thing that 

 is constantly changing. Seeing that there are so many 

 difficulties in the method, it is to be regretted that Fizeau 

 never published the details of his observations, so that we 

 might judge of their agreement with each other. 



We now come to a branch of the subject of enormous 

 importance. I have already said that there were two 

 theories to account for the propagation of light. The only 

 evidence in favour of the undulatory theory, was that it 

 explained a large number of facts that either could not be 

 accounted for by the corpuscular theory, or else were ex- 

 plained in a clumsy way that was too artificial to merit 

 belief. 



This was a very important piece of work, but a great 



