418 An Essay on the Re/lection, Refractiony and Ivjlection 



he has cleansed the Augean stable of all its impurities, aixi no 

 hypothesis now can long hold its ground whose foundation is not 

 laid on the basis of experiment. 



Newton was one of the first of the philosophers that conform- 

 ed entirely to this principle, and made experiment the ground- 

 work of his philosophy ; and it is upon this basis that I have 

 grounded all my opinions. I shall propose nothing but what I 

 can prove by analogy to be at least possil)le, or what I shall show 

 can be more satisfactorily explained upon my principles than 

 in any other way. There can now be no danger of my propa- 

 gating error, even if my opinions should be wrong, because, in that 

 case, they will not stand the test of experiment ; and moreover, 

 where my opinions differ from those which have hitherto been 

 received, if the argument should not be manifestly on my side, 

 prejudice will always be against me, the name of Sir Isaac New- 

 ton and the preconceived opinions of all the philosophers will 

 add weight to the opposite scale, and I can hope for no victory 

 but in the confidence of having truth on my side, which, sooner 

 or later, must inevitably succeed. 



Rays of hght on being emitted from any luminous body are 

 propagated in straight lines ; but on the intervention of a me- 

 dium, — a glass lens for instance, — they are affected in two very 

 remarkable ways : those rays that pass through are refracted, or 

 bent out of their course by a power that we call refraction, while 

 the remainder are turned back by the surfaces of the lens by a 

 power which is called reflection. 



I shall show presently that Sir Isaac Newton's hypothesis of 

 alternate refraction and reflection is not borne out by analogy, 

 and that the cause he has assigned for it is not adequate to the 

 efl"ect: but first of all I shall explain what is my own opinion 

 concerning the cause of these phenomena. 



As these two phaenomena of reflection and refraction are to- 

 tally dissimilar, it appears to me to be highly probable that they. 

 are produced by two distinct causes; that is, that there are two 

 distinct substances in the medium, one of which possesses in its 

 nature the power of reflecting light, and the other that of re- 

 fracting it. When I speak of a medium however as possessing 

 these properties, I am to be understood with some limitation. 

 I believe glass, water, and every thing else that is called a me- 

 dium, to be perfectly neutral in these respects, and that the 

 power of reflecting and refracting light resides in a fluid or gas, 

 composed of two distinct substances, that adheres to the surface 

 of these bodies. This I shall prove, first, by showing that these 

 phaenomena cannot be explained by supposing that these powersi 

 reside in the bodies themselves 3 secondly, by showing that there, 



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