POLARISATION OF LIGHT. 



thod of getting over the small double 

 refraction of rock crystal has been used 

 by Dr. Wollaston. 



"Let ABCDa&cdEFGH efgh 

 (Jig. 20.) be two halves of a hexagonal 

 prism of quartz (the form it affects), 

 produced by a section parallel to two of 

 the sides. In the vertical face A DC? a 

 draw any line L K parallel to the sides, 

 and therefore to the axis of the prism, 

 (which is also that of double refraction,) 

 and join C L, c k. 



" Then a plane CLkc will cut off a 

 prism C L K d c D, having L k, D d, or 

 C c, for its refracting edges, either of 

 which is parallel to the axis. Again, in 

 the other half of the prism join E/ 1 and 

 H g, and cut the prism by a plane 

 passing through these lines; then, re- 

 garding either portion as a double 

 refracting prism, having for refracting 

 edges the lines E H,fg, tfose will have 

 the axes of double refraction perpendi- 

 cular to their refracting edges ; and, in 

 particular, the axes will lie in the faces 

 HEeA, or ~FGgf at right angles to 

 H E orfg. If, then, we take care to 

 make the refracting angle C L D of the 

 prism CLKrfcD equal to that of the 

 edge H E of the prism H E efg h ; and 

 if we make these two prisms act in 

 opposition to each other, placing the 

 edge H E opposite to D d, and the edge 

 h e opposite to K L ; and having thus 

 brought the two surfaces DLftd and 

 HE eh in contact, cement them toge- 

 ther with mastic, or Canada balsam, it 

 is evident that their principal sections 

 will be at right angles to each other; and 

 therefore only two images will be formed, 

 the whole of the extraordinary ray of 

 the one prism passing into the ordinaiy 

 image of the other, and vice versa. Now, 

 to see how this acts to double the sepa- 

 ration of the images, let us conceive m n 

 to be a luminous line viewed through 

 one of the prisms, with its edge down- 

 wards and horizontal. It will be sepa- 

 rated into two images, e and o, the one 

 more raised than the other. Suppose 

 the ordinary image to be most refracted. 

 Then, if we interpose the other prism 

 with its edge upwards, both these 

 images will be refracted downwards ; 

 but the ordinaiy image o, which was 

 before most raised, now undergoing ex- 

 traordinary refraction, is least depressed, 

 and comes into the position o e, while the 

 extraordinary one e, which was before least 

 raised, is now most depressed, and comes 

 into the situation e o ; and it is evident, 

 that (the refracting angles being equal,and 



the double refraction of the two prisms 

 the same) the line o e will fall as far 

 short of the original line m n, as e o sur- 

 passes it, viz. by a quantity equal to the 

 distance between the two first images 

 o and e ; so that the distance between 

 the twice refracted images, is double 

 that of those which have undergone only 

 one refraction*." 



8. Single Image Prisms of Iceland 

 Spar. Prisms of this kind were first 

 used by Dr. Brewster. The method 

 of making them is to roughen as 

 much as possible the two surfaces, or 

 even one surface, of a prism of Iceland 

 spar, and to cover it with grooves. A 

 fluid or balsam with the same refractive 

 index as the ordinary ray, is then 

 placed between the rough surface of the 

 spar and a plate of glass. This polishes, 

 as it were, the surface for the ordinary 

 ray, and allows it to pass through un- 

 interrupted, in consequence of the spar 

 and the fluid having the same index of 

 refraction for that ray. The extraor- 

 dinai-y ray, on the other hand, is scat- 

 tered in all directions by reflection at the 

 separating surfaces of the grooves and 

 the fluid, and totally disappears, leaving 

 only the ordinaiy image. But as there 

 is no proper oil or fluid of such a high 

 refractive power as 1.654, it is better 

 to take an oil of the same refractive 

 index as that of the extraordinary ray, 

 for the surface which is roughened. If 

 the oil does not exactly suit the surface, 

 a slight inclination of the prism one way 

 or another, will produce the adjustment. 

 When this is done, we shall see the ex- 

 traordinary image quite distinct, while 

 the ordinary image has wholly disap- 

 peared. For ordinary purposes this 

 prism is perfectly sufficient, but for 

 others it will not answer so well, as the 

 nebulous light seen all round, is polarised 

 in a plane opposite to that of the ex- 

 traordinary image. 



9. Agate Plates and Microscopes. 

 Among the bodies of the mineral king- 

 dom, Dr. Brewster found agate to be 

 one which gave only one distinct image, all 

 the light of which is polarised in one plane. 

 He therefore used it in his experiments. 

 Agate microscopes, or plates of agate 

 placed close to a single microscope, may 

 be very advantageously used. 



10. Tourmaline Plates. M. Biot and 

 M. Seebeck discovered that certain yellow 

 ish tourmalines, that is, those which are 

 yellowish by refracted light, transmitted 

 only one pencil polarised in the same 



Mr. Herschei's Treatise on Light. 



