crystallized bodies on homogeneous light. 91 
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 perfect, 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 phenomena, however, are entirely independent of the first 
and principal cause which produces the deviation in crystals 
with two axes, viz. the separation of the axes of differently 
coloured rays, and are referable solely to the secondary and 
subordinate cause, of which Rochelle salt has just afforded an 
example, viz. a peculiarity 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, fol- 
lowing the same order, whatever be the thickness of the plate. 
It follows from this, that the multiplier M in our general 
formula, (a) is the same for all the coloured rays, being zero 
at the commencement of the scale; and hence it follows, as 
a necessary consequence, that the axes of all the colours are 
united in one, and the virtual and actual poles coincide with 
each other and with the centre. Did any sensible separation 
of the axes exist, it must become perceptible by the ellipticity 
of the rings when examined with homogeneous light of that 
colour from which they are farthest asunder ; but with the 
greatest attention, in plates of considerable thickness, I have 
not been able to observe the slightest shifting of the axis, or 
deviation from the circular figure, in passing from a red to a 
