. 3 33^35' 



respecting Sound and Light. 125 



X. Of the Analogy between Light and Sound. 



Ever since the publication of Sir Isaac Newton's incom- 

 parable writings, his doctrines of the emanation of particles of 

 light from lucid substances, and of the formal pre-existence 

 of coloured rays in white light, have been almost universally 

 admitted in this country, and but little opposed in others. 

 Leonard Euler indeed, in several of his works, has advanced 

 some powerful objections against them, but not sufficiently 

 powerful to justify the dogmatical reprobation with which he 

 treats them ; and he has left that system of an ethereal vibra- 

 tion, which after Huygens and some others he adopted, equally 

 liable to be attacked on many weak sides. Without pretending 

 to decide positively on the controversy, it is conceived that some 

 considerations may be brought forwards, which may tend to 

 diminish the weight of objections to a theory similar to the 

 Huygenian. There are also one or two difficulties in the New- 

 tonian system, which have been little observed. The first is, 

 the uniform velocity with which light is supposed to be pro- 

 jected from all luminous bodies, in consequence of heat, or 

 otherwise. How happens it that, whether the projecting force 

 is the slightest transmission of electricity, the friction of two 

 pebbles, the lowest degree of visible ignition, the white heat of 

 a wind furnace, or the intense heat of the sun itself, these won- 

 derful corpuscles are always propelled with one uniform velo- 

 city ? For, if they differed in velocity, that difference ought to 

 produce a different refraction. But a still more insuperable 

 difficulty seems to occur, in the partial reflection from every 

 refracting surface. Why, of the same kind of rays, in every cir- 

 cumstance precisely similar, some should always be reflected, 



