IN CRYSTALLIZED MEDIA. 325 



Now I conceive that this is by no means a fair charge, for if the 

 material particles themselves vibrate, we have two systems of vibrating 

 particles, the combined motion of which has been considered, and if 

 they do not vibrate, they produce no effect. 



As regards the law of force, a Memoir has lately been circulated, 

 in which M. Cauchy arrives at the conclusion that it is the inverse 

 fourth power of the distance. Adopting this law. Professor Lloyd has 

 proved that the vibrations are transversal, in a paper read before the 

 Irish Academy, in November last. In a short abstract of that paper, 

 it is stated that the object of the Author has been simplification, and 

 the mode of accomplishing that object is given. This mode is precisely 

 that which I adopted, and some of the conclusions are apparently the 

 same ; as for instance, that the vibrations are transversal. This con- 

 clusion is stated as follows : " When this law of force (the inverse 

 fourth power) is substituted in the corresponding relation for the normal 

 vibration, the velocity of propagation is infinite; so that the normal 

 disturbance is propagated instantaneously, and gives rise to no wave." 



I do not think from this statement that the grounds on which the 

 law of the inverse square stands, are less tenable than those which lead 

 to the inverse fourth power, and shall not therefore consider it incum- 

 bent on me to change my views with respect to the law. 



I have dwelt at considerable length on this point, as it is of essen- 

 tial importance to all my succeeding investigations that the law of the 

 inverse square of the distance be not set aside; and I think it will 

 be allowed, that as far as the above speculations are concerned, that 

 of the inverse fourth power does not appear to be established. 



In attempting to offer any investigations connected with the trans- 

 mission of light through crystals, we are naturally prompted to recur, 

 as to the established theory, to those of M. Fresnel, which stand pro- 

 minent as an example of clearness of conception and distinctness of 

 explanation. The agreement of the results with those of observation, 

 tlie remarkable predictions which they have afforded of phenomena 

 which have fully verified those predictions, the simplicity with which 



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