" On the hflexion of Light" .425 



of science, I trust the pages of this Journal will not be consi- 

 dered as improperly occupied by a few remarks, which have 

 suggested themselves to me on reading Mr. Barton's paper 

 above referred to. 



I hope also that Mr. Barton will have no ground to com- 

 plain of me as a prejudiced " undulationist," determined at all 

 hazards to support a favourite theory. I am desirous that 

 every theory should be examined with fairness and impar- 

 tiality. But it appears to me, in the present stage of this in- 

 quiry, that in preference to pulling down or constructing 

 theories in toto, it would be far the more philosophical mode 

 of proceeding, to examine carefully the extent of the actually 

 demonstrated laws to which the varied phenomena of light 

 are referrible, and thence to pursue the inquiry as to what 

 sort of hypothetical action will best account for the greatest 

 number of them. Of such action some of the characteristics 

 of periods or intervals appear so indisputably essential to the 

 explanation of the results, that no one, capable of appreciating 

 the accumulated and cumulative evidence on which the assump- 

 tion of them rests, can doubt the legitimacy of that assumption. 

 And these characteristic actions are precisely those which 

 would necessarily result from the vibrations of an aether. In 

 the present state of our knowledge if we should allow that 

 this sort of theoretical action may fail to account for several 

 facts, yet we must contend that it unquestionably accounts for 

 a vast number more than any other principle which has been, 

 or probably can be, alleged. 



(1.) With regard to the author's first objection I shall merely 

 observe, that on any theory we must admit the reality of the 

 intervals, which are called lengths of undulations. And in in- 

 terference-experiments, let us suppose two rays arrive at the 

 centre of the screen in a conspiring state, and give a bright 

 point ; the effect at other points on the screen will depend 

 upon the successive differences in the length of route of the two 

 rays arriving together at those points, compared with that of 

 the first pair of rays. It is not the absolute, but the relative 

 differences of route with which we are concerned, and these 

 are measured from the aperture as the origin. 



But on the undulatory view of the matter, it is a point be- 

 longing to the most unequivocal and elementary part of the 

 theory, that any small portion of a large wave may be taken, 

 separately from the rest, as the origin of a new small wave di- 

 verging from it in a spherical form. Such an origin of a small 

 wave (in Mr. Barton's diagram) occurs at A, where, if the 

 aperture be very small, it is shown by theory that the new wave 

 diverging from it, which is produced by the sum of all the 



Third Series. Vol. 2. No. 12. June 1833. 3 I 



