-kV% Mr . Hersehel on the Action of [March, 



and consequently that, even were its axes coincident, its tints, 

 though perfectly regular, would still differ very sensibly from the 

 colours of thin plates. This secondary cause of deviation ought 

 to become sensible in plates cut so as to contain both axes,^ if 

 examined at a perpendicular incidence ; but I have not yet had 

 an opportunity of making the trial. 



If we calculate on the numbers above given, it will soon appear 

 that a perfect coincidence of all the colours in a single virtual pole 

 is impossible. For this purpose we may employ our equation (?) 

 which easily affords the following 



COS, 2.(<t 4- d) « COS. 2 a ^l -\- j^-j, . tan. 2 a . sin. ( — I d)l 



COS. 2 a 

 "~ (cos. M)« 



taking M an auxiliary angle such that 



tan. M'= "^ 2 . tan. 2 a . y-^ . sin. (— J a) 



whence the value of 5 or the position of the coincidence of any 

 two coloured rays becomes known, the values of /, V, and — S « 

 being given from the foregoing tables. If we unite the mean 

 red with the mean green, these formul-ae give d = -- 11° 29', and 

 if with the mean blue, 6 = — 14° 8', of which the one falls 

 short of, and the other exceeds the angle — 13° 1' given by 

 observation. If we determine by interpolation the values of /'' 

 aad — . Sa, which give d = ~ 13° 1', we shall find very nearly 



V = 34581 - J « = 3° 3r - g fl + 1° IV- = 4° 39' 



which correspond to a blue ray strondy inclining to green, and 

 in the brightest part of the colour. iSow it is evident that when 

 a rigorous union of all the rays in the proportion in which they 

 exist in white light, is impossible, that of the strongest and 

 brightest colours in opposition to each other will at least ensure 

 the nearest approach to a virtual pole on the principles above 

 demonstrated, and a white will thus be produced, not indeed 

 mathematically periact, but containing no marked excess of any 

 of the more powerful colours. 



The apophyllite is the only crystal with one axis whose tints 

 exhibit a sensible deviation from the scale of Newton. Its phe- 

 nomena, however, are entirely independent of the first and prin- 

 cipal cause which produces the deviation in crystals with two 

 axes, viz. the separation of the axes of diflferently coloured rays, 

 and are referable solely to the secondary and subordinate cause, 

 of' which Rochelle salt has just afforded an example, viz. a pecu- 

 liarity in the law which regulates the lengths of the minimum 

 oscillations of the differently coloured rays within the medium. 



1. The tints of the apophyllite commence at the centre of the 

 rings, and increase in regular progression outwards, following the 

 same order, whatever be the thickness of the plate. 

 ^ It follows from this, that the multiplier M in our general for- 



