54 



POLARISATION OF LIGHT. 



Fig. 62. 



rated by the line O P of no double refrac- 

 tion. This system of fringes is exactly 

 the same as that produced by a rect- 

 angle of glass heated and then rapidly 

 cooled, and when it was crossed by a 

 rectangle of glass having the system of 

 fringes developed by the passage of heat 

 across it, the intersectional rectangle 

 exhibited the elliptic fringes mentioned 

 in p. 51. A plate of jelly one-third of 

 an inch thick, and one inch and two- 

 thirds long, produced at r s a green of 

 the second order on the day after it was 

 coagulated. 



When isinglass is placed in a cylin- 

 der AB of glass, Jig. 63, open at both 

 ends, and is allowed to indurate, it ap- 



Fig. 63. 

 A 



All the lenses of animals which are 

 formed of albuminous matter exhibit a 

 doubly refracting structure analogous 

 to that of spheroids of glass and indurated 

 jelly. In some of them there is only a 

 single structure, as in that of man and 

 several quadrupeds ; but in the lens of 

 the horse there are three structures, like 

 those in plates of glass. In fishes, which 

 have spherical or rather spheroidal 

 lenses, the phenomena are highly beauti- 

 ful and instructive. If we take the 

 crystalline lense of a large cod, and 

 having immersed it in a glass trough of 

 oil, place its axis parallel to A C,/g\ 39, 

 we shall see the beautiful figure shown 

 mfig. 64, which will never vary while 



Fig. 64. 



I 



pears to be divided into two structures 

 o, p, by a dark line in n, when A B is 

 parallel or perpendicular to the plane 

 of primitive polarisation. The tints of 

 o descend in the scale, while those of p 

 ascend, when the axis, x y, of a plate of 

 sulphate of lime crosses them as in the 

 figure. 



If we take a sphere or spheroid of 

 transparent jelly and allow it to indu- 

 rate, it will exhibit the same phenomena 

 as a sphere or spheroid of glass that has 

 received the doubly refracting structure, 



the lens is turned round upon its axis. 

 The figure consists of twelve luminous 

 sectors 1, 2, 3, 4, 5, &c., separated from 

 each other by a black cross, and two dark 

 concentric circles, which are circles of no 

 double refraction. The interior sectors, 1 , 



2, 7, 8, are small, and exhibit a white tint 

 of the first order, increasing in brilliancy 

 towards the centre where the black 

 cross is very sharp. The middle sectors, 



3, 4, 9, 10, which are very large, are 

 separated from the interior by a broad 

 dark circle, and display a white tint of 

 the first order. The outer sectors, 5, 6, 

 11, 12, are extremely faint, and are seen 

 with considerable difficulty in this po- 

 sition of the lens. If the axis of the lens 

 (or the axis of vision of the eye) is inclined 

 to the polarised ray in a plane passing 

 through 1 and 2, the sectors 1 and 2 will 

 diminish, and 7, 8 will increase in size, 

 and an additional luminous space will 

 appear at the centre, till, by increasing 

 the inclination, the sectors 1 , 2 and the 

 luminous space will completely disap- 

 pear, leaving the sectors 7, 8 much en- 

 larged, and of a' bluish white tint. If the 

 lens is inclined in a plane passing 

 through 7 8, the sectors 1, 2 will in- 

 crease, and 7, 8 diminish in the same 

 manner 



