146 Sir D. Brewster's OOservatiotis on M. Rudberg's Memoir, 



n" 

 We thus obtain 0-99560. My experiments give — p = 0*99542, 



which is 0*000 18 in defect. These differences, however, evi- 

 dently arise, on the one hand, from the difficulty of determining 

 these ratios by experiments on refraction with prisms different- 

 ly cut, with a pi'ecision comparable to that which we obtain by 

 means of experiments on diffraction; and on the other hand 

 from the probable inaccuracy in the value of the inclination of 

 the optical axes found by the observation of the coloured rings. 



XXXII. Observations 071 the preceding Memoir. 5j/ Sir David 

 Brewster, K.H. LL.D. F.R.S. V.P.R.S. Ed. 



lyOTWITHSTANDING the great accuracy and value of 

 -^ the preceding observations, and the importance of the 

 deductions which the author has drawn from them, yet we are 

 constrained to state, that almost all the general principles at 

 which M. Rudberg has arrived have been previously esta- 

 blished by English philosophers, though not by observations 

 made by means of the fixed lines in the spectrum. 



The variation of the inclination of the optic axes with the 

 different colours of the spectrum, and the increase of that 

 angle with the refrangibility of the colour in some crystals, 

 such as Arragonite, and its decrease with the refrangibility in 

 other crystals, such as topaz, is the discovery of Sir John F. W. 

 Herschel, and one of the most important that has been made 

 on the subject of double i-efraction ; and yet the name of Sir 

 John Herschel is not once mentioned. Sir John indeed did 

 not examine Arragonite and topaz, but he found the very same 

 phaenomena in sidphate ofbarytes and Rochelle salts ; and as I 

 had myself discovered that all those crystals in which the in- 

 clination of the optic axes increases with the refrangibility, have 

 the red ends of their systems of rings inwards, or towards the 

 axis A ; while those in which this inclination decreases with the 

 refrangibility have the red ends of their rings outwards, or to- 

 wards the axis B, and had determined that Arragonite had the 

 red ends of its rings inwards, and topaz the red ends outwards*; 

 the variation of the inclination bf the optic axes in these two mir 

 nerals, and its inverse character, were both known before 

 M, Rudberg's experiments. To M. Rudberg, however, there 

 remains the merit of having given the values of these angles, 

 ^nd that too in reference to fixed points of the spectrum. 



It is impossible to overlook the great difference betweeij, 

 theory and observation in the inclination of the optic axes of 

 Arragonite and topaz as given by M. Rudberg. His observaiil 



Art. Optics, in the Edinburgh Eiicyclopcedia, vol. xv. p. 596- 



