194 JSfew Properties of Light. [March, 



tartrate cf potash aiul soda, and sulphate of potash, I concluded that 

 their conjugate diameters were inversely as {m — 1)', m being the 

 index of refraction; J3ut the nitrate of potash forms a singular 

 anomaly, as it ])roduces in the direction of the axis of the hexaedral 

 prism a series of miniature rings nearly einht times less than they 

 should have been, according to the preceding law. They are, how- 

 ever, inversely as the thickness of the crystal. These miniature 

 rings form one of the finest phenomena in optics. 



IV. Polarisation of Li^ht by Reflection. 



1. By Reflection Jrom Iraitsparevt Surfaces. — The polarisation 

 of a pencil of light by reflection from transparent bodies, discovered 

 by JNIalus, is not, as he supposed, a general law, but seems to 

 depend upon the rckition between the quantities of reflected and 

 transmitted light when the pencil is incident at the polarising 

 angle. As this angle increases with the refractive power of the 

 body, and as th.e quantity of -reflected light also increases with the 

 angle of incidence, the pencil reflected at the polarising angle from 

 realgar is more than one half of the incident light. The conse- 

 quence of this is, that the realgar is incapable of polarising the 

 whole of' the pencil. The same thing happens in the case of 

 diamond and chromate of lead ; and we are thus led to our views 

 respecting the state of the rays before their approach to the 'refract- 

 ing surface. 



2. By Reflection from Oxides of Steel, — When a pencil of light 

 incident at an angle of 6S° upon a blue oxidated surface is viewed 

 through a prism of calcareous spar, the images become alternately 

 of a lieautiful red colour, without any intermixture of blue, in eveiy 

 quadrant of the circular motion of the prism. The blue light, 

 therefore, is polarised at that particular angle, and the red light 

 which is transmitted tlirough the film alone reaches the eye of the 

 observer. At a greater angle of incidence, the red image becomes 

 orange, and gradually approaches, as the angle increases, to the 

 colour of the other image ; at less angles of incidence than 63°, the 

 red image becomes blue, and at last assimilates itself to the other 

 image. Does it not follow from this experiment, taken in conjunc- 

 tion with other facts, that a part of the light v.-hich metals reflect 

 has penetrated their surface, and has thus received the peculiar 

 colour of the metal, '.riie transparency of gold leaf, the green 

 colour of the liglit which it transmits, and its power of polarising a 

 jiortion of light liy refraction, as I liave fomid from direct experi- 

 ment, confirms thi« conjecture, and would also lead us to conclude 

 that the part of tlie light which has penetrated the metal must be 

 polarised in an opposite plane to that which has nut entered the 

 body. It follows also from this experiment, that the refractive 

 power of the blue oxide is about 2'0(), a result almost the same as 

 that which Dr. Thomas Young deduced from a very diflerent 

 inethod. 



