Astronomical and Nautical Collections, 115 



identical with it] ; and these undulations, as well as the waves 

 of water, are subject to the laws of interference. 



The undulations formed in the interior of a fluid have here 

 been mentioned in a general manner : in order to form a dis- 

 tinct idea of this mode of propagation, it must be remarked, 

 that when the fluid has the same density and the same elas- 

 ticity in every direction, the agitation produced in any point 

 must be propagated on all sides with the same velocity : for 

 this velocity of propagation, which must not be confounded 

 with the absolute velocity of the particles, depends only on 

 the density and elasticity of the fluid. It follows thence that 

 all the points, agitated at the same instant in a similar man- 

 ner, must be found in a spherical surface, having for its centre 

 the point which is the origin of the agitation : so that these 

 undulations are spherical, while the waves, which are seen on 

 the surface of a liquid, are simply circular. 



We give the name oirays to the right lines drawn from the 

 centre of agitation to the different points of this spherical 

 surface ; and these rays are the directions in which the motion 

 is propagated. This is the meaning of the term sonorous 

 rays in acustics, and oiluminous rays or rays of light in the 

 system which attributes the phenomena of light to the vibra- 

 tions of a universal fluid, to which the name of ether has been 

 given. 



The nature of the different elementary motions, of which 

 each wave is composed, depends on the nature of the different 

 motions which constitute the primitive agitation. The sim- 

 plest hypothesis that can be entertained concerning the form- 

 ation of the luminous undulations, is, that the small oscilla- 

 tions of the particles of the bodies, which produce them, are 

 analogous to those of a pendulum removed but little from its 

 point of rest ; for we must conceive the particles of bodies, 

 not as immoveably fixed in the positions which they occupy, 

 but as suspended by forces which form an equilibrium in all 

 directions. Now, whatever the nature of such forces may be, 

 as long as the displacement of the particles is but small in 

 proportion to the extent of their sphere of action, the accele- 

 rating force which tends to restore them to their natural po- 

 sition, and which thus causes them to oscillate on each side of it, 

 may always, without eensible error, be considered as propor- 



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