R^r action of differently-coloured Rays in Apophyllite. 1S9 
the ordinary pencil on one side of the axis took place. These 
are as follows : 
Extreme Red. 
Violet. 
19° 
16^ 
23° 
55' 
19 
48 
25 
27 
19 
5 
25 
39 
19 
48 
25 
34 
19 
5 
26 
31 
20 
10 
25 
46 
19 
5 
25 
55 
19 
5 
25 
27 
19 
5 
26 
54 . 
. near the Indigo. 
— 
25 
16 . 
. extreme Violet. 
Mean 19° 
28' 
25° 
31' 
The variations in the observations of the violet rays arise not 
so much from the imperfections of the specimen, and the diffi- 
culty of taking measures in violet light, as from the rapid change 
of the polarising energy, as the ray approaches the extremity of 
the spectrum, and the mean result above set down is, in conse- 
quence, that corresponding nearly to the mean violet rays. I 
now interposed between the crystal and the reflector, on which 
the incident light received its polarization, a plate of mica, so 
thin as to polarize (alone) a bluish-white of the flrst order ; its 
plane being perpendicular to the ray. My object in using so 
thin a plate, was to dilate or contract the system of rings, by^ 
quantity decidedly smaller than half the interval between two 
contiguous ones, so as to avoid all possibility of a mistake in the 
order of the ring brought under examination, by the interposi- 
tion of the mica plate ; the point in question being not to obtain 
any precise numerical results, but merely to ascertain whether 
the change of the inclination corresponding to the minimum or 
maximum of any given ring, produced by a mica plate of in- 
finitesimal thickness, would have the same or opposite signs for 
the two ends of the spectrum, 
The mica being fixed so as to have its principal section 45° 
inclined to the plane of primitive polarization (or in azimuth 
45°), the minima of the ordinary pencil were now observed, to 
take place at the following inclinations. 
