in Reply to Professor Powell. 175 



ground " truly straight,") facing each other, and their points 

 meeting at a given small angle, he computed the interval be- 

 tween them at any distance from that angle, by the problem 

 of similar triangles. This method appears susceptible of 

 greater accuracy than any process of direct measurement. 

 If, therefore, Fresnel employed some such direct means, I 

 should be disposed to place greater confidence in the observa- 

 tions of Newton, who was no less remarkable for accuracy as 

 an experimentalist than for genius as a mathematician. Pro- 

 fessor Airy, indeed, finds Newton's measurements bad, be- 

 cause, " according to theory, when the breadth of the aper- 

 ture is very small, the distance should vary as the square of 

 the breadth," which is not found to be the case in Newton's 

 observations. But I would ask, Is not this taking for granted 

 the very thing to be proved ? When we are seeking to bring 

 to test the soundness of a theory by comparing it with the re- 

 sults of observation, can it be allowable to measure the ac- 

 curacy of those very observations by their accordance or dis- 

 cordance with the theory in question ? 



The results obtained by Newton and Biot display, I think, 

 strong internal evidence that their discrepancy with the theory 

 of Fresnel is not attributable solely to errors of observation. 

 It will be observed, on examining those results, that when the 

 width of the slit exceeds the thirtieth part of an inch, or there- 

 abouts, the observations are not greatly at variance with 

 the theory; but in proportion as the width is reduced below 

 this limit, the discordance between theory and observation 

 increases ; and at length, when the width is less than the four- 

 hundredth part of an inch, it becomes a difference, not only 

 in degree, but in kind ; the theory indicating that the spectrum 

 should be brightest at the centre, while observation shows 

 that this centre is quite dark. Looking at this progressively 

 increasing discrepancy between the conclusions of theory and 

 the observed phenomena, it is difficult to avoid suspecting 

 that it depends on some defect in the theory itself. 



In addition to these objections to the theory of Fresnel, 

 founded on its want of agreement with observed phaenomena, 

 I suggested, in my former communication*, an objection of 

 another kind. I endeavoured to show that the origin of 

 the undulations supposed to constitute light is placed in this 

 theory at a wrong point Fresnel and his followers placed this 

 origin at the point of intersection— whether a pin-hole through 

 which the rays enter the darkened chamber, or the focus of 

 a lens occupying the same position. To me it appears that 



* See Lond. and Edinb. Phil. Mag. vol. ii. p. 263.— Edit. 



