200 Reflection and Refraction of Light. 



incapable of any satisfactory solution on these grounds ; that the new 

 vibrations should be propagated both ways till the incidence becomes a 

 certain quantity, and then that they should all cease entirely in the new 

 medium and be excited with a much greater intensity in the first, presents 

 the appearance of a paradox which cannot put on the garb of truth. 



] 7. The seventh fact, viz. that of the dispersion of light, or its unequal 

 refrangibility, is allowed to be inexplicable, and to present a difficulty 

 which has not been removed, and probably never can be surmounted ; at- 

 tempts however are making to clear up this very important point, by in- 

 volving in the calculation some functions of the displacements of the 

 particles of the vibrating ether, which till now have been neglected : how 

 far this may affect the explanations of other phaenomena, or accord with the 

 facts themselves to be explained, I cannot tell ; but altogether the theory 

 is remarkable for a multitude of twistings. 



Besides all this, the results in general depend on the assumption that 

 the elasticity of the ether, and consequently the velocity of light is less in 

 bodies which are of a greater refractive density. This is said to be pro- 

 bable ; but no good reason is assigned to shew its probability : I think it 

 is improbable, exceedingly improbable, if not impossible. The particles 

 of bodies have no action on the ether, or they act on it either by attraction 

 or repulsion. Now if they have no action on it, the pores of bodies will 

 be occupied by the ether in the same state as in a vacuum, the elasticity 

 will be unaffected, and there can be no reflection or refraction. If the 

 actions between the particles of bodies and those of the ether were only 

 attractive or repulsive, no diminution of elasticity could arise on that ac- 

 count; indeed if any alteration could take place from either of these cir- 

 cumstances, we should expect an increase of elasticity in the denser body. 

 Hence this supposed diminution of elasticity, and consequently of velocity 

 in the denser medium, if admitted at all, must be admitted gratuitously, 

 since no good physical cause can be assigned for such a change. 



18. In explaining reflection and refraction, it has been assumed, that 

 in passing from a rare to a dense medium, we must consider small elastic 

 bodies as striking larger, and from a dense to a rarer medium, large par- 

 ticles must be considered as striking smaller ones : but this applies only 

 by a far-fetched analogy ; we have, in this instance, not merely simple 

 lines of elastic bodies, we must take into the account the essential property 

 of fluidity. 



In closing this part of the subject, we may just notice, that even after 

 the enormous admission, which allows space to be uniformly occupied by 

 an universal fluid, the cause seems inadequate to the effect. We have no 

 reason to think that the light emitted from the several parts or points of 

 the sun's surface is more intense than that of a lamp or common candle, 

 and even if the elasticity of the ether were perfect, the expanding waves 

 so feebly excited, would, as we should naturally conclude, before reaching 



