216 Mr. Moon in Reply to Jesuiticus. 



Double Refraction is based. It rests on a lower level still. 

 The true basis of the theory is, that the eethereal medium 

 consists of particles separated by finite intervals (to use a well- 

 known, but improper mode of expression), and acting upon 

 each other by their mutual attractions. From this principle, 

 the so-called fundamental hypothesis of Jesuiticus is a suffi- 

 ciently easy inference : I have thought it necessary to remark 

 upon this inaccuracy, however, as from the extraordinary want 

 of precision of the writers on this subject, it is somewhat dif- 

 ficult to say what is their real starting-point ; at the same time, 

 that in order to make a proper estimate of Fresnel's theory, 

 and of the skill and judgement with which he has worked it 

 out, it is very desirable that that fact should be clearly ascer- 

 tained. 



Jesuiticus afterwards goes on to say, "It is then proved, that 

 if any particle of the eether be suddenly displaced, the other 

 particles remaining quiescent, the force of restitution developed 

 by such disturbance will not in general be in the direction of 

 the displacement, but only when such displacement is in the 

 direction of the aforesaid axes of elasticity. The elegant de- 

 monstration of Mr. Smith, quoted by Mr. Moon, is by Mr. 

 Moon's own showing fully adequate to establish the theorem 

 as I have enunciated it, which is doubtless the sense in which 

 Fresnel conceived it." 



I admit that Mr. Smith's demonstration is fully adequate to 

 establish the theorem as Jesuiticus has enunciated it, but I 

 must beg to assure Jesuiticus, that unless the demonstration 

 establishes a great deal more than the theorem so enunciated, 

 it is not, for the purpose for which it is adduced, worth the 

 paper it is written upon. What is the use of considering the 

 impossible case of a single particle suddenly disturbed while 

 all the other particles remain quiescent, and then reasoning 

 upon what takes place in the beginning of the motion in that 

 case, as if the same held good throughout the whole motion in 

 the actual case, when all the particles are vibrating together, 

 when it is perfectly certain that it does not? 



Jesuiticus says, "Any one who understands the subject 

 must at once acknowledge that any theory of light must be, 

 to a considerable extent, imaginative ; and that theory which 

 can explain the greatest number of facts ought to claim the 

 attention of the philosopher more than any other." Of the 

 justice of the remark contained in the first part of the above 

 sentence, Fresnel's theory is no doubt a remarkable confirma- 

 tion ; in the sentiment of the second clause of it I am disposed 

 to concur, with the reservation that some portion of the credit 

 due to a theory depends on its antecedent probability. But 



