of Newton's Rings. 423 



tan(t'-t") . 2tt 2ir . _, 



- — r; — «r «sm •— (#£ — a-) - 6«cos — (vt — x) , 

 tan (i + 1 ) X ' x v ' ' 



where b is always small but never = 0, and is perhaps constant. 



The conclusions at which I have arrived are at variance with 

 one of Sir David Brewster's (Phil. Trans. 1815). Sir David Brew- 

 ster's character as an experimental philosopher stands deservedly 

 so high, and my estimation of his accuracy, (as observed by 

 myself in the repetition of many of his experiments) is so great, 

 that I think it necessary to point out distinctly the nature of 

 this disagreement. 



Sir David Brewster states that homogeneous light is com- 

 pletely polarized by the diamond at the proper angle. I have 

 made no experiments here with homogeneous light, and I know 

 that, on account of its extreme faintness however obtained, little 

 confidence can be placed in results which depend only on the 

 evanescence of the reflected light. But the phenomena observed 

 by me are entirely inconsistent with this supposition. If homo- 

 geneous light were used, then (on this supposition) the bright- 

 centered rings would disappear and black centered rings would 

 succeed them as at the polarizing angle of the glass. If white 

 light were used, the rings in the neighbourhood of the polarizing 

 angle would be wholly coloured, and on changing the angle the 

 intensity of the different colours in each ring would alter, but 

 there would be nothing like contraction. Thus at a certain angle 

 the brightest part of the red would be at the center of the spot, 

 and its faintest part would be in the first ring; while for the 

 blue the places would be reversed : on increasing the angle the 

 brightest parts of both would be in the first ring. Whereas in 

 my experiments there was no discoverable alteration in the 

 colours of the rings, there never was seen a bright red center 



