by Rough and White Surfaces. 349 



As in ground glass, one or more plates of glass inclined to 

 the reflected ray, as previously described, brought the neutral 

 point in almost all these substances to the A or N side of G. 



When the substance was more or less glazed, the neutral 

 point was on the A side of G, the glazed surface acting exactly 

 like the polished surface of a plate of ground-glass when placed 

 next the light. This effect is finely seen in milk, where the 

 fluid surface increases the negative or refracted light as in the 

 glass plate. 



I attempted to determine for several of these substances the 

 angle of maximum polarization, the intensity of the partial polari- 

 zation, and the place of the neutral point ; but I found it very 

 difficult, owing to the magnitude of the flame which was neces- 

 sary to show the bands when very faint, and to its proximity to 

 the reflecting surface, which was necessary for the same purpose. 



In white unglazed paper, for example, in the sun's light on 

 the 1st of February 1841, at ll h a.m., the polarizing angle was 

 about 71°, and the degree of polarization, or R = 18^°. On the 

 2nd of February the polarizing angle was 69i°, and R = 18J° 

 at 10 h 40 m A.M. 



In almost all the substances which I have examined, I have 

 observed a neutral point only at angles of incidence below the 

 maximum polarizing angle, and beyond G, fig. 2 ; but it is ob- 

 vious that there must be another at some angle above the maxi- 

 mum polarizing angle, excepting in substances where the light 

 polarized by refraction is too feeble to neutralize the light po- 

 larized by reflexion. I have observed this second neutral point 

 only in one case ; but with a sufficiently strong light it will 

 doubtless be seen in many substances. 



In all the preceding experiments, the substances employed 

 have been opake, or with such rough surfaces that objects 

 cannot be seen through them. I was therefore desirous of 

 ascertaining if neutral points were produced when the surfaces 

 which polarize the light were perfectly transparent, having some 

 analogy with the strata of the atmosphere of different densities. 

 With this view I used a pile of transparent plates with twenty 

 surfaces, and I found a neutral point distinctly visible, both 

 above and below the angle of complete polarization. I obtained 

 the same result at an angle less than that of complete polariza- 

 tion, with a large plate of mica split by an intense heat into so 

 many films that it had the metallic lustre of silver. 



The results now submitted to the Society could hardly have 

 been anticipated from theoretical considerations. The laws of 

 polarization for light normally reflected and refracted by po- 

 lished surfaces, are not applicable to those which are rough, or 



