Experiments and Inquiries rejpelfing Sound and Light, 87 



electricity; and the arguments againft the exiftence of fuch an ether throughout the 

 univerfe, have been pretty fufficiently anfwered by Euler. The rapid tranfmiflion of the 

 eleflrical fhock, fliows that the eleftric medium is poffeffed of an elafticity as great as is 

 neceffary to be fuppofed for the propagation of light. Whether the cleftric ether is to be 

 confidered as the fame with the luminous ether, if fuch a fluid exifts, may perhaps at fome 

 future time be difcovered by experiment ; hitherto I have not been able to obferve that the 

 refraftive power of a fluid undergoes any change by eledtricity. The uniformity of the 

 motion of light in the fame medium, which is a difEculty in the Newtonian theory, 

 favours the admiffion of the Huygenian; as all imprelTions are known to be tranfmitted 

 through an claftic fluid with the fame velocity. It has been already Ihown, that found, in 

 all probability, has very little tendency to diverge : in a medium fo highly elaftic as the 

 luminous ether mufl; be fuppofed to be, the tendency to diverge may be confidered as in- 

 finitely fmall, and the grand objr;£tion to the fyftem of vibration will be removed. It is 

 not abfolutely certain, that the white line vifible in all direftions on the edge of a knife, 

 in the experiments of Newton and of Mr. Jordan, was not partly occafioned by the ten- 

 dency of light to diverge. Euler's hypothefis, of the tranfmiflion of light by an agitation 

 of the particles of the refraifting media themfelves, is liable to fl:rong obje£lions ; accord- 

 ing to this fuppofition, the refra6tion of the rays of light, on entering the atmofphere from 

 the pure ether which he defcribes, ought to be a million times greater than it is. For ex- 

 plaining the phsenomena of partial and total refleflion, refraftion, and inflexion, nothing 

 more is neceflary than to fuppofe all refrafting media to retain, by their attraftion, a 

 greater or lefs quantity of the luminous ether, fo as to make its denfity greater than that 

 vhich it poflefl[es in a vacuum, without increasing its elallicity.; and that light is a propa- 

 gation of an impuife communicated to this ether by luminous bodies : whether this im- 

 pulfe is produced by a partial emanation of the ether, or by vibrations of the particles of- 

 the body, and whether thefe viirations are, as Euler fuppofed, of various and irregular' 

 magnitudes, or whether they are uniform, and comparatively large, remains to be here- 

 after determined. Now, as the direftion of an impuife tranfmitted through a fluid, 

 depends on that of the particles in fynchronous motion, to which it is always perpen- 

 dicular, whatever alters the dircdlion of the puife, will infle£l the ray of light. If a 

 fmaller elafttc body ftrike againft a larger one, it is well known that the fmaller is refle£led 

 more or iefs powerfully, according to the difference of their magnitudes: thus, there is 

 always a refledlion when the rays of light pals from a rarer to a denfer ftratum of ether ; 

 and frequently an echo when a found ftrikes againft a cloud. A greater body ftriking a 

 fmaller oncj propels it, without lofing all its motion : thus,, the particles of a denfer 

 ftratum of ether, do not impart the whole of their motion to a rarer, but, in their effort to 

 proceed, they are recalled by the attra£tion of the refra(fiing fubftance with equal force}, 

 and thus a refle£lion is always fecondarily. produced, when the rays of light pafs from a 

 denfer to a rarer ftratum. Let AB, Plate V. Fig. 29, be a ray of light fallings on , the 

 nsfle^ting, furface FGj (^ the diredlion . of . the vibration, pulfe, impreffion,.or,cQtlden- 



fatiop.1. 



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