Proceedings of the British Association. 265 



spectrum so that the rays which pass by the edge of the plate may 

 interfere with those which pass through it, then if the plate is on the 

 same side as the violet space, the spectrum is seen crossed with 

 numerous black and nearly equidistant bands, parallel to Fraunhofer's 

 fixed lines, and, generally speaking, increasing with the thinness of 

 the plate ; but if the plate is on the same side as the red space, no 

 bands whatever are seen, though all the other conditions of their 

 production are the same. When the transparent plate is very thin 

 the fringes of thin plates are produced, whether we cover the half 

 or the whole of the pupil ; but these have nothing to do with the 

 phenomena under consideration. The singular fact of the fringes 

 being seen only in one position of the plate appeared to me to indi- 

 cate a new polarity in the simple elements of light. I therefore 

 communicated it to the British Association at Liverpool, in 1836 ; and 

 in 1837 I submitted to the same body additional observations, which 

 excited some discussion. The singular phenomena contained in these 

 notices, though pressed upon the attention of the supporters of the 

 Undulatory Theory, remained unexplained for more than three years. 

 They at last attracted the regard of Prof. Airy, in October, 1839, 

 when that distinguished mathematician repeated my experiments; 

 and in 1840 he made them the subject of an elaborate memoir, con- 

 stituting the Bakerian Lecture of that year, entitled, c On the Theore- 

 tical Explanation of an apparent New Polarity in Light/ (Sir D. 

 Brewster read the parts of Prof. Airy's paper which could be readily 

 understood by the section.) Previous to the publication of this in- 

 genious paper, Prof. Airy gave an account of it at the meeting of the 

 British Association in Glasgow, in 1839. On that occasion I made 

 a few observations upon it ; but especially marking the fact, that 

 whereas Prof. Airy's explanation referred solely to very faint bands 

 seen when the spectrum was out of focus, I had seen the bands per- 

 fectly distinct, and most vivid and intensely black, when the spectrum 

 was in focus. The explanation, therefore, given in this memoir had 

 nothing to do with the bands which I had discovered and described. 

 Prof. Airy was accordingly led to resume the investigation ; and he 

 has published the results of it in a Supplement to his first paper, 

 which appeared in the Philosophical Transactions for 1840. The 

 following is the account which he gives of the results which he 

 obtained : — 



