SECT, xx.] VIBRATIONS OF ETHER. 191 



motion of each particle of the rope, which never deviates 

 far from a state of rest. So, in ether, each particle vibrates 

 perpendicularly to the direction of the ray ; but these vibra- 

 tions are totally different from, and independent of, the un- 

 dulations which are transmitted through it, in the same 

 manner as the vibrations of each particular ear of corn are 

 independent of the waves that rush from end to end of a 

 harvest field when agitated by the wind. 



The intensity of light depends upon the amplitude or ex- 

 tent of the vibrations of the particles of ether ; while its colour 

 depends upon their frequency. The time of the vibration of 

 a particle of ether is, by theory, as the length of a wave 

 directly, and inversely as its velocity. Now, as the velocity 

 of light is known to be 190,000 miles in a second, if the 

 lengths of the waves of the different coloured rays could be 

 measured, the number of vibrations in a second corresponding 

 to each could be computed. That has been accomplished as 

 follows : All transparent substances of a certain thickness, 

 with parallel surfaces, reflect and transmit white light ; but, 

 if they be extremely thin, both the reflected and transmitted 

 light is coloured. The vivid hues on soap-bubbles, the iri- 

 descent colours produced by heat on polished steel and copper, 

 the fringes of colour between the laminae of Iceland spar and 

 sulphate of lime, all consist of a succession of hues disposed 

 in the same order, totally independent of the colour of the 

 substance, and determined solely by its greater or less thick- 

 ness a circumstance which affords the means of ascertaining 

 the length of the waves of each coloured ray, and the fre- 

 quency of the vibrations of the particles producing them. If 

 a plate of glass be laid upon a lens of almost imperceptible 

 curvature, before an open window, when they are pressed 

 together, a black spot will be seen in the point of contact, 

 surrounded by seven rings of vivid colours, all differing from 

 one another (N. 194). In the first ring, estimated from the 

 black spot, the colours succeed each other in the following 

 order : black, very faint blue, brilliant white, yellow, orange, 



