1B21.] Crystallized Bodies Dti Homogeneous Light, 'f7[l 



stances for ascertaining this important point, the law of propor- 

 tionality seems to be sustained with great precision. This may 

 seem to authorize the general conclusion, that in all cases^ 



~ s= -. Let us see how this agrees with the measures given ill 



the former part of this paper. 



In sulphate of baryta, if we take Dr. Brewster's measure 6? 

 the dispersive power,* we have S R = 0*019, an<i consequently, 

 calculating on the data determined in p. 161, we must have, at 

 the virtual pole, 



^ = 21° 6' 30" ' 9' = 20° 50' 34)'' 3^ = - 15' 



:^ow, if we suippose7 = 6-3463 V = 3*9982, the vMues 6t6 

 and c' respectively for the extreme red and violet rays,t we shall 

 find by substitution in our formula (A) 



la= 51' 10" 

 But a red ray penetrating the surface from within the crystal at 

 an angle a = 18° 12' 30", and a violet one at an angle n +-§--« 

 = 19° 3' 40'', would emerge at the respective angles 30° 59' and 

 32° 58' 20", and would include between them an angle of 

 1° 59' 20'', which should be the apparent separation of the red 

 and violet axes in the plate employed. Now, previous to the 

 computation of this result, I had carefully measured this angle, 

 'by observing the incidences at which the extreme red and violet 

 rays of the prismatic spectrum, received on the reflector of a 

 graduated apparatMS, respectively disappeared from the extraor*- 

 Jdinary image at the poles P, P'. I thus found 



Interval of the poles P, P' for red rays 62° 2' 



Ditto for violet QQ 5 



Semi-difference, or apparent separation of the axes, 2° 1' 30", 

 -which differs from its computed value only by 2' 1 0". We may, 

 therefore, fairly conclude, that in the case of sulphate of bar}^ta, 



the hypothesis - = - does not deviate sensibly from the truth. 



^If we apply our formula (i) to the measures above given f^Mr 

 Rochelle salt, the result will be widely different. The same 

 supposition as to the values of/, r being made, we get 



§ a = 4° 2' 50'' : 



The incidence being nearly perpendicular, and the angle smaU, 

 we need only increase it in the proportion 1*499 : 1, to have the 

 apparent angle, which thus comes out 6° 4'. We have already 

 found 9° 46' for the same angle, by a method which must neces- 

 sarily give a result much below the truth. This difference is by 

 far too great to arise from any errors of observation ; but to 

 obtain more exact measures, I took several times the apparent 



* 8 R = 019. Treatise on new Philosophical Instrument». i 



f Biot, Traite de Physique, vol. iv. 



