R^r action qf differently-coloured Rays in Apopliyllite, 141 
nlzzzt . tan d 
which gives sin « = {s! ^ \ ^ pi -p), where » = 
we find that it will take place at 38° 12' of inclination, and the 
variations produced by the mica in the place of the ordinary mi- 
nimum being -|- 5° 4' and 3° 36', are decidedly less than the 
interval between the maximum and minimum of the first ring, 
which is 38° 12' - 26° 19' = 11* 53'. 
However, to leave no doubt on this head, I interposed an ex- 
tremely thin plate of mica, which polarized but a feeble blue of 
the first order, and placing its principal section successively at 
azimuths 45° and 45° 4-90° = 135°, repeated the measures of 
the incHnation at which the first minimum took place, taking in 
each case the mean of ten observations, to compensate the va- 
rious irregularities it was found impossible to avoid. I thus 
obtained the following results. 
1 
Azimuth — 45° 
Azimuth — 135° 
Inclination at Minimum of Extreme Red, 
Ditto Violet, - - - - 
18° 39' 
26 15 
19° 55' 
24 35 
The feeble double refraction, joined to the imperfection of 
the specimen, and the impracticability of cutting it into a prism, 
with its refracting edge parallel to the axis, (owing to the facili- 
ty with which the laminae separate from each other,) have pre- 
vented my verifying these results by actual observations on the 
double refraction. There can be no doubt, however, what would 
be the result. The spectra formed by the two refractions would 
appear superimposed on each other, but of different lengths, the 
indigo spaces of each coinciding. They would consequently ap- 
pear as one spectrum of diluted colours, and unless examined in 
homogeneous light, the double refraction would not be noticed 
at all. 
There can be little doubt, too, that the third variety of apo- 
phyllite described in my paper, would exhibit similar phenome- 
na; but I have not thought it necessary to make any experi- 
ments on it, the fact which it is the object of this paper to point 
out, being, I hope, sufficiently proved without it. 
Slough, 
March 31. 1821. 
