AND ON THE COLOURS EXHIBITED BY CERTAIN FLAMES. 459 



to the angle subtended by the interval between the corres- 

 ponding edges AB of the slits ; and this being known, the dis- 

 persive power is easily calculated. In fact, if i be the interval 

 between the corresponding edges of the slits observed, and d 

 their distance from the prism, A the angle of the prism, D the 

 deviation of the extreme red rays, p the refractive index for 

 those rays, and p the dispersive power, we have 



cos 



P = ^£j where V = V 



(^) 



sin 



d 



2 



For example, to determine the dispersions of 8 specimens of 

 glass, I found by this method as follows : 



i i ~ 0.535 



Number, &c. of Glass. 



A = 



39° 40' 



D = 



ft = 



d = 



J (& ■=. 



P = 



1. Flint, 



26° 8' 



1.6010 



34.40 



0.03849 



0.06404 



2. Do. 



30 36 



18 36 



1.5780 



49.76 



0.03705 



0.06409 



3. Do. 



25 5 



15 10i 



1.5847 



61.95 



0.03734 



0.06386 



4. Do. " Heavy," - 



24 



14 56 



1.6028 



61.40 



0.03951 



0.06555 



5. Flint, 



24 15 



14 38 



1.5847 



64.08 



0.03747 



0.06409 



6. Crown, 



40 6 



23 1 



1.5265 



58.60 



0.02139 



0.04063 



7. Do. a different kind, 



24 40 



13 29| 



1.5301 



94.90 



0.02494 



0.04704 



8. Plate, 



24 45 



13 51 



1.5133 



102.30 



0.02616 



0.05096 



These results will doubtless appear extraordinary, after all 

 that has been written on the subject of dispersive powers. 

 The highest value of _p, for flint-glass, given in Dr Brewster's 

 Table, is only 0.052, which is ^th lower than any of these re- 

 sults ; yet I have no doubt of the correctness of my observa- 

 tions. The agreement of the dispersions of the three first spe- 

 cimens of flint, within a 260th part of their whole quantity, 

 while the refractive indices differ so considerably, is not a little 

 remarkable. The prisms used were made at once, by the same 



artist, 



