49 
crystallized bodies on homogeneous light. 
in which he expresses himself otherwise than obscurely on its 
cause, he appears to regard the deviated tints as analogous 
to those developed along the axis of rock crystal and by cer- 
tain liquids ; an analogy which, in the present state of our 
knowledge on that perplexing subject, it seems not easy to 
admit. In a paper too, which has lately appeared, containing 
the interesting observations of the same excellent philosopher 
on the optical structure of the apophyllite, he remarks the 
very striking deviation of the colours of this crystal from 
Newton’s scale “ in the first orders” of its rings; and while 
he remarks that such deviations are common enough, and 
indeed universal in crystals “ in which the rings are formed 
by the joint action of two axes,” seems to think this analogy 
close enough to authorize the substitution of two rectangular 
axes of a negative character for the single positive axis 
actually observed, according to his own peculiar and ingeni- 
ous views on this subject. T lost no time in endeavouring to 
procure a specimen of this mineral, and by the kindness of 
my friend, Sir Samuel Young, (to whom I owe more than 
one obligation of this nature) was favoured with one sufficiently 
transparent for optical examination. From my observations 
on this body, I think I shall be able to demonstrate satisfac- 
torily, that the phenomena of the apophyllite depend on a 
principle distinct from that which produces the chief part 
of the deviation of tints in most crystals with two axes. 
The course I propose to pursue is, first, to describe the 
phenomena themselves. I shall then show how these phe- 
nomena, complicated as they are in appearance, are all redu- 
cible to one very simple and general fact ; viz. that the axes 
MDCCCXX. H 
