BY KOUGH AND WHITE SURFACES. 209 



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 maxi- 

 mum polarisation, the intensity of the partial polarisation, and the place of the 

 neutral point ; but I found it very difiicult, owing to the magnitude of the flame 

 which was necessary to show the bands when very faint, and to its proximity to 

 the reflecting surface, which was necessary from the same cause. 



In white unglazed j)aj)er for example, in the sun's light on the 1st of February 

 1841-, at ll'' A.M., the polarising angle was about 71°, and the degree of polarisa- 

 tion, or R,=18i°. On the 2d February the polarising angle was 69i°, and R=18i° 

 at 10'' 40"! 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 polarising angle, 

 and beyond G, fig. 2 ; but it is obvious that there must be another at some angle 

 above the maximum polarising angle, excepting in substances where the light 

 polarised by refraction is too feeble to neutralise the light polarised 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 opaque, 

 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 sur- 

 faces which polarise 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 dis- 

 tinctly visible, both above and below the angle of complete polarisation. I 

 obtained the same result at an angle less than that of complete polarisation, 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 polarisation for light, normally 

 reflected and refracted by polished surfaces, are not applicable to those which are 

 rough, or to bodies which reflect light from their interior ; and in the case of piles 

 of polished glass-plates, the most distinguished philosophers, Aeago, Young, and 

 Sir John Herschel, would have considered the light which the plates reflected, 

 as they did that which they transmitted, as consisting of polarised light, accom- 

 panied with a portion of common light, a combination incapable of producing 

 neutral points. 



VOL. xxiii. part ti. 3 M 



