crystallized bodies on homogeneous light, 47 
while, on the other hand, the splendid phenomena of the 
polarised rings, which we owe to Dr. Brewster, have es- 
tablished the connection of the tints so polarised with the 
force producing the deviation of the extraordinary pencil, and 
shown the legitimacy of conclusions respecting the intensity 
of the latter, drawn from observations on the former. 
This indirect mode of observation, which consists in notic- 
ing the gradations of colour for different positions and thick- 
nesses of the crystal, possesses three capital advantages. The 
first is its extreme sensibility, which enables us to detect the 
existence, and measure precisely the intensity of forces, far 
too feeble to produce any measurable deviation of the extra- 
ordinary pencil. It, in fact, affords the rare combination of an 
almost indefinite enlargement of our scale of measurement, 
with a possibility of applying it precisely to the object mea- 
sured, arising from the distinctness of all its parts. Another, 
no less precious, is the leading us by mere ocular inspection 
to the laws of very complicated phenomena, and enabling us 
to form, and mould as it were our analytical formulas, not on 
a laborious, and sometimes deceptive discussion of tabulated 
measures, but on the actual form of the curves themselves, 
which are loci of the functions under consideration. It is 
true, that a reference to tabulated measures is indispensible to 
give precision to such first approximations ; but the power 
this mode of observation affords of copying our outline fresh 
from nature, and from the general impression of the phe- 
nomena, brought at once under our view, is an advantage 
not to be despised. Nor ought we, lastly, to omit, in our 
estimate of advantages, the means thus afforded us of sub- 
jecting the minutest fragments of a crystal to a scrutiny as 
