TRANSACTIONS OF THH SECTIONS. 13 



On the Dichroism of the Palladio-chlorides of Potassium and Ammonium. 

 By Sir David Brewster. 



Dr. Wollaston had found that a long crystal of either of these salts, when looked 

 through transversely, had a green colour, but when looked through from either end, 

 had a red colour; and he (Sir D. Brewster) placed one of these long crystals trans- 

 versely over another, in a cruciform shape, and then found that those portions of the 

 centres of both, which were in contact, gave a red colour, while all the ends of the 

 crystals were red. 



On the existence of a New Neutral Point, and two Secondary Neutral Points. 

 By Sir David Brewster. 



After noticing the two neutral points (points where there is no polarization of light) 

 of MM. Arago and Babinet, Sir David Brewster said he had discovered a third. 

 He also mentioned amongst some general results of observations continued for a long 

 time, that instead of the point of maximum polarization being always, as supposed, at 

 90° from the sun, he had found it more frequently 88° from the sun. He also de- 

 scribed a polarimeter or polariscope, by which, he said, the rectilinear bands in polar- 

 ization were seen more clearly than by other methods. 



On certain Cases of Elliptically Polarized Light. By Prof. Powell. 



At the last meeting of the Association, Prof. Lloyd* gave a theoretical investigation 

 of certain results obtained by Sir D. Brewster relative to thin films from which polar- 

 ized light is reflected. Besides completely explaining those results, Prof. Lloyd infers, 

 that such films ought to give the portions of light reflected at their two surfaces differ- 

 ing in phase, and that the light should be consequently in general elliptically polarized. 



The author of the present paper, before he was aware of the investigation of Prof . 

 Lloyd, had made many observations on the elliptical polarization of light by reflexion 

 from metallie and other surfaces, — the method of observation being by the well-known 

 dislocation of the polarized rings. Some of these experiments went merely to prove 

 the existence of elliptic polarization in cases where it had not previously been detect- 

 ed, as in certain minerals and other bodies in which it is seen though of small amount. 

 In other cases the reflecting surface consisted of the thin films formed on polished 

 metal by tarnish, by heat, or by the galvanic process of Nobili. In these instances, a 

 verification was afforded of Prof. Lloyd's theory by direct observation. But, further, 

 these films give periodic colours ; and in passing from one tint to another, the ellip- 

 ticity, as disclosed by the form of the rings, underwent regular changes, passing from 

 a dislocation in one direction to the opposite, through points of no dislocation or of 

 plane polarization, the rings being alternately dark and bright centred. This afford- 

 ed a further field for the application of theory, and Mr. Airy investigated a formula 

 for the rings under these varying conditions, with which the phenomena are in perfect 

 accordance. 



On Crystalline Reflexion. By Sir David Brewster. 



Having (said Sir David), in a conversation with Prof. Kelland, had my attention 

 directed to Prof. M'Cullagh's interesting memoir on the laws of crystalline reflexion 

 and refraction, I have felt it necessary to make a communication on the subject to 

 the British Association. In consequence of the results which I laid before the Bristol 

 meeting, Prof. M'Cullagh was led to revise the views to which he had been led by my 

 earlier experiments in 1819. I had at that time the advantage of communicating with 

 him personally and by letter ; and, having preserved copious abstracts of his paper on 

 the subject, I did not look into the memoir itself till yesterday, when my attention 

 was drawn to the following note : — " I was at this time in doubt whether the phe- 

 nomena observed with oil of cassia could be reconciled to that theory ; and when the 

 note in page 36 was written, I was almost certain that they could not. But I have 

 since, I think, found out the cause of this perplexity : some of Sir David Brewster's 

 experiments were made with natural surfaces of Iceland spar ; others with surfaces 



* See Report, 1841. Transactions of the Sections, p. 26. 



