^178 Mr, Hers^kel.onthe Actiqnof [Mai^Cih, 



refractive forces is given, or vice veysuy the dispersive powers .may 

 Jte obtained from the angles of the resultant axes for the red aijd 



^^yiolet rays of the spectrum. 



J " 9. 1 have found crystals in^which these phenomena are cif^ct- 



^Jdedly comiected with the rotatory phenomena ; and from this 



;^iglily important fact I am led to conclude that both have the 

 same origm, and that all the rotatory phenomena are, as I have 

 stated in my paper, the result of the uncompensated tints of two 

 axes, equal for the mean ray, but unequal for all the rest. 



^^{llere follows an illustration bj/ a diaphragm.) 



" 10. The diviision into two classes m sect. iii. as founded 

 merely on observation, is converted into another division into 

 two classes ; viz. 1. That in which the doubly refracting force of 

 the principal axis acts more powerfully on the blue rays than the 



c^other axis does ; and 2. That in which it acts less powerfully, 

 i'he first class comprehends those crystals in which the blue ends 

 are inwards, and the second, those in which , the TQd.eftds^re 

 inwards, or nearer the principal axis." 

 In a subsequent letter (Oct. 4), he adds, 



"The virtual poles, which you mention, I discovered in the 

 year 1815, and I have two accounts of them in my Journal, the 

 one signed on the 24th January, 1816, and the other 6th Janu- 

 ary, 1817, by Sir G. Mackenzie, President of the Physical Class 



;jt)f the Royal Society." 



No comments on the above extracts are necessary. They 

 establish at once the priority of Dr. Brewster's observations, and 

 the independence of mine. With regard to the division of crys- 

 tals into two classes, which observation has alike suggested to 

 both of us, it is unnecessary, if we regard either of the two 



jii^lasses as having the angle. between the resultant axes greater 

 than a right angle. In Dr. Brewster's table, Phil. Trans. 1818, 

 p. 230, succinic acid and sulphate of iron are stated as having 

 this angle 90*^. If this determination corresponds, as in all pro- 

 bability it does, to the yellow rays, they belong at once to both 



/Classes, and are, in fact, instances of the hmit where one class 

 passes into the other. Bicarbonate of ammonia, in which L can 



^perceive no separation of tlie axes of different colours, nor of 

 course, any virtual poles, belongs in like manner to both classes, 

 or to neither. John F. W. Herschel. 



APPENDIX. 



^Description of an Instrument employed in the foregoing Experi^ 

 ments on the polarned Rings. 



'The singular property possessed by the tourmahne, by which 

 a plate of it of any moderate thickness cut in a direction parallel 

 to its axis of double refraction, is enabled to absorb the whole, or 



