18 PROPAGATION OF LIGHT. 



in a given time, from the vibrating particles of the ether, 

 the sensation of violet being produced by the most frequent 

 vibrations, and that of red by the least frequent. But the 

 number of vibrations performed in a given time varies inversely 

 as the time of a single vibration ; the colour of the light, 

 therefore, varies with the time of vibration, or with the length 

 of the wave in a given medium. By experiments, which will 

 be described hereafter, it has been found that the length of a 

 wave, in air, corresponding to the extreme red of the spectrum, 

 is 266 ten-millionths of an inch, and that corresponding to the 

 extreme violet 167 ten-millionths. The length of the wave 

 corresponding to the ray of mean refrangibility is nearly 200 

 ten-millionths, or J^OTJU^ ^ an i ncn 



It appears, then, that the sensibility of the eye is confined 

 within much narrower limits than that of the ear ; the ratio 

 of the times of the extreme vibrations which affect the eye 

 being only that of 1*58 to 1, which is less than the ratio of the 

 times of vibration of a fundamental note and its octave. There 

 is no reason for supposing, however, that the vibrations them- 

 selves are confined within these limits. In fact, we know that 

 there are invisible rays beyond the two extremities of the spec- 

 trum, whose periods of vibration (and lengths of wave) must 

 fall without the limits now stated to belong to the visible 

 rays. 



(21) The aberration of the light of the fixed stars results 

 from the movement of the Earth in its orbit, combined with the 

 movement of light. Nothing can be simpler than its expla- 

 nation in the theory of emission. In fact, we have only to 

 combine the two co-existing motions according to the known 

 mechanical law, and the apparent direction of the star is that 

 of their resultant. The angle between this direction, and that 

 of the principal component, is called the aberration. 



In order to explain this phenomenon, in accordance with 



