crystallized bodies on homogeneous light. 51 
while the law ofdispersion remains unknown. But we may go 
yet farther. The nature of the formula furnishes an equation 
by which the actual quantity of the separation of the extreme 
red and violet axes may be deduced from observations of 
the tints of a very simple and accurate nature, being perfectly 
analogous in principle to the “ method of coincidences/' which 
has of late been applied with such success to the most delicate 
investigations in every department of physical science. The 
comparison of the results afforded by that equation, with those 
deduced by direct observation on homogeneous light, while it 
leaves nothing to desire in point of accuracy, leads to another 
important result, viz. that the proportionality of the minimum 
lengths of the periods performed by differently coloured mo- 
lecules, in a doubly refracting crystal to the lengths of their 
fits of easy reflection and transmission, supported as it is by 
an induction of no ordinary extent and accuracy, is yet not 
universal, admitting a deviation to a very large amount. 
Hence must of course arise a kind of secondary deviation in 
the scale of tints. In crystals with two axes, however, this is 
masked by the much more powerful effect of the separation 
of the coloured axes ; yet even there, is not altogether insen- 
sible in an extreme case. In the apophyllite, however, the 
agency of this secondary cause is placed in the fullest evi- 
dence, The application of our general formula to the ano- 
malous tints of that body, while it proves incontestably the 
exact coincidence of the axes for all the coloured rays, points 
out at the same time a peculiarity in its action on the more 
refrangible extremity of the spectrum, of a nature so sin- 
gular, so entirely without example in all the multitude of 
