♦1821.] Crystallized Bodies on Homogeneous Light, 177 



Xor perhaps to a certain small degree in all cases, the minimum 



^lengths of the periods deviate in their respective proportions 

 from those of the fits of easy transmission and reflection ; a cir- 



^cumstance which of itself is sufficient to prove the independence 

 of the causes of these laws of periodicity. If we take R r = 

 R A, fig. 6, and construct a curve whose abscissas A P are the 

 values of c, c', &c. and ordinates those of /, /', &c. the straight 

 line r y g b i V inclined at 45° to A R will represent the locus 



• for crystals, such as carbonate ofHme, in which tlie periods fol- 

 low the Newtonian law, / o^ j/' g' V i' v' will represent the same 

 locus for tartrate of soda and potash, while \ r" o" y" g" b" i" v" is 

 the curve similarly traced for apophyllite. 



Having communicated to Dr. Brewster my observations on 

 the deviation of tints, and the conclusion I had thence deduced 

 as to the separation of the axes of the differently coloured rays, 

 I received in answer a letter, from which, injustice to that inde- 

 fatigable observer, I subjoin the following extracts : 



" MY DEAR SIR, Esk Hill, hy Roslin Laswade, Styt. 18, 1819. 



" In consequence of having been some time from home, I 

 have only now received your letter, and hasten to reply to that 

 part of it in which you request me to state what results I had 

 obtained respecting the deviation of the tints from Newton's 

 scale. The following general points will enable you to judge of 

 the progress which I had made in the inquiry. 



^^ 1. in almost all crystals with two axes there is a deviation 

 from the tints of Newton's scale. 



" 2. This deviation is greater in some crystals than in others, 

 being a maximum in acetate of lead and tartrate of potash and 

 soda. 



" 3. That all these crystals may be divided into two classes, 

 viz. those which have the red ends of the rings inwards and the 

 blue ends outwards, and those which have the red ends outwards 

 and the blue ends inwards. 



" 4. That in all crystals with two axes, the doubly refracting 

 force of one axis in general acts differently upon the differently 

 coloured rays from the doubly refracting force of the second axis 



^'5. That as the polarising force is always proportional to the 

 force of double refraction, the polarising force of one axis will 

 act differently on the differently coloured rays from the polaris- 

 ing force of the other axis. 



<^ 6. * * -^^ * ^ * * # * 



'^ 7. The consequence of this is, that there will be different 

 resultant axes, or different points of compensation for the differ- 

 ently coloured rays. 



*' 8. All these effects may be calculated with the utmost accu- 

 racy, if the ratio of the dispersive powers of the two extraordinary 



New Series, vol. i. m 



