TRANSACTIONS OF THE SECTIONS. IS 



On the Combined Action of Grooved Metallic and Transparent Sur- 

 faces upon Light. By Sir D. Brewster. 



The phenomena described in this paper, discovered by the author, 

 were altogether new and of a very remarkable description. The spectra, 

 produced by the methods which were explained to the meeting, were 

 covered with bands liiie those produced by the action of nitrous gas 

 upon the spectrum, and the phenomena varied with the distance of tlie 

 grooves, with the relation of the dark and luminous intervals, and with 

 the inclination of the incident ray. Sir David Brewster described 

 analogous phenomena and others of a remarkable character when the 

 grooves were made in transparent surfaces ; and he explained to the 

 Section the manner in which he conceived the phenomena were pro- 

 duced, on the principles of interference. 



On a New Kind of Polarity in Homogeneous Light. 

 By Sir D. Brewster. 



At the last meeting of the Association Sir D. Brewster communi- 

 cated an account of a new property of light, which did not admit of 

 any explanation. Since that time he has had occasion to repeat and 

 vary the experiments; and having found the same property exhibited 

 in a series of analogous though diiferent phenomena, he has no hesi- 

 tation in considering this property of light as indicating a new species 

 oi polarity in the simple elements of light, whether polarized or unpo- 

 larized. In the original experiment, two pencils of perfectly ho- 

 mogeneous light, emanating from the same part of a well-formed 

 spectrum, interfered after one of them had been retarded by trans- 

 mission through a thin plate of glass. The fringes were exceedingly 

 black, but no phenomena of colour were visible. He was anxious to 

 observe what would take place when the retarded pencil passed through 

 the edges of various plates differing very little in thickness, so that dif- 

 ferent parts of it suffered different degrees of retardation, for the pre- 

 ceding experiment entitled him to expect a series of overlapping bands 

 and lines of different sizes. In making such an experiment, however, 

 he encountered great difficulties, and he failed in every attempt to com- 

 bine such a series of thin edges. He had recourse therefore to lami- 

 nated crystals, and in an accidental cleavage of sulphate of lime he ob- 

 tained the desired combination of edges. " Upon looking through this 

 plate at a perfect spectrum, in the manner described in my former com- 

 munication, I was surprised to observe a splendid series of bands and 

 lines crossing the whole spectrum, and shifting their place and changing 

 their character by the slightest inclinations of the plate. But what sur- 

 prised me most was to perceive that the spectrum exhibited the same 

 phenomena as if it had been acted upon by absorbing media, so that 

 we have here dark lines and the effects of local absorptions produced 

 by the interference of an unretarded pencil with other pencils, proceed- 

 ing in the same path with different degrees of retardation. The bear- 

 ing of this unexpected result upon some of the most obscure questions 



