S32 ON A NEW STKUCTUUE IN APOPHYLLITE, AND 



the Other Positive axis, or the Vertical one perpendicular to 

 the laminae, and the system of rings round the resultant of these 

 two axes, will deviate more or less from Newton's scale, ac- 

 cording to the nature of the dispersive forces of the elementary 

 axes *. Let us suppose that the resultant negative vertical axis 

 has the same action upon the Yellow rays of the spectrum, as 

 the real positive vertical axis ; but that it acts much more ener- 

 getically upon the Red extremity, and much less energetically 

 upon the Blue extremity of the spectrum. The yellow rays 

 being thus solicited by equal and opposite forces, the crystal 

 will exercise over them no polarising energy. The led rays 

 being subjected to a greater polarising energy from the Nega- 

 tive than from the Positive axis, will give rings corresponding 

 to the difference of their opposite actions, and the characters 

 of these rings will be Negative. The blue rays, on the contra- 

 ry, being much less energetically acted upon by the Negative 

 than by the Positive axis, will form rings proportional to the 

 difference of their actions, and these rings will be Positive, 

 from the predominating influence of the positive axis. In this 

 way, a particular crystal of Apophyllite may exercise over the 

 red rays of the polarised beam a negative influence ; over the 

 blue rays a positive influence ; and over the yellow rays no in- 

 fluence at allf ; while it is the general character of the mineral to 

 exert an attractive doubly refracting force over all the rays of 



the 



♦ For an illustration of these views, the reader is referred to my letter to Mr 

 Herschel, published at the end of his paper in the Phil. Trans. 1820, p. 94.; 

 and to Mr Heeschel's paper in the Transactions of the Cambridge Philosophical 

 Society, vol. i. ; or in the Edin. Phil. Journal, vol. iv. p. 335. ; and vol. v. p. 340. 



•f- This partial equilibrium of polarising forces is analogous to the paradoxical 

 phenomena of a compound lens, which, as I have elsewhere shewn, may be con- 

 structed so as to converge the Bine rays, diverge the Red rays, and exercise no ac- 

 tion at all upon the Yellow ones. That is, the same compound lens is a Plane lens 

 in yellow light, a convex one in blue light, and a concave one in red light. 



