April 27, 1871] 
NATURE 
315 

plate S, shall proceed vertically upwards. N is a Nicol’s prism 
or any other analyser, placed in the path of the second reflection. 
The diaphragm is furnished with a ring, moveable in its own 
plane, by which the crystallised plate to be examined may be 
placed in any azimuth. C isa small moveable stand, by means 
of which the film to be examined may be placed in any azimuth 
and at any inclination ; for the usual experiments this is removed. 
If a lamina of quartz cut parallel to the axis, and sufficiently 
thin to show the colours of polarised light, be placed upon the 
diaphragm so that its principal section (é.2. the section containine 
the axis) shall be 45° to the 4/f of the plane of reflection, on 
turning the analyser from left to right, instead of the alternation 
of two complementary colours at each quadrant, which appear 
in the ordinary polarising-apparatus, the phenomena of successive 
polarisation, exactly similar to those exhibited in the ordinary 

apparatus by a plate of quartz cut perpendicular to the axis will 
be exhibited: the colours follow inthe order R, O, Y, G, 
B, P, V, or, in other words, ascend as in the case of a right- 
handed plate of quartz cut perpendicularly to the axis. If the 
lamina be now either inverted, or turned in its own plane 99°, so 
that the principal section shall be 45° to the right of the plane of 
Teflecticn, the succession of the colours will be reversed, while 
the analyser moves in the same direction as before, presenting the 
same phenomena as a left-handed plaie of quartz cut perpen- 
dicularly to the axis. Quariz is a positive doubly refracting 
crystal, and in it consequently the ordinary index of refraction is 
smi ller than the extraordinary index. But if we take a lamina 
of a negative crystal, in which the extraordinary index is the 

least, as a film of Iceland spar split parallel to one of its natural 
cleavages, the phenomena are the reverse of those exhibited by 
quartz ; when the principal section is on the //f of the plane of 
reflection the colours descend, and when it is on the vig/t of the 
same plane the colours ascend, the analyser being turned from 
left to right. 
It has been determined that the ordinary ray, both in positive 
and negative crystals, is polarised in the principal section while 
the extraordinary ray is polarisedin the section perpendicular 
thereto. It is also established that the index of refraction is in- 
versely as the velocity of transmission. It follows from the above 
experimental results, therefore, that when the resolved ray whose 
plane of polarisation is to the left of the plane of reflection is the 
quickest the successive polarisation is right-handed, and when it 
is the slowest the successive polarisation is left-handed ; in the 
order R, O, Y, G, B, P, V, and in the second case in the re- 
versed order, 
The rule thus determined is equally applicable to laminz of 
diaxal crystals, 
As selenite (sulphate of lime) is an easily procurable crystal, 
and readily cleavable into thin laminze capable of showing the 
colours of polarised light, it is most frequently employed in ex- 
deriments on chromatic polarisation. The Jaminz into which 
this substance most readily splits, contain in their planes the two 

optic axes ; polarised light transmitted through such laminz is 
resolved in two rectangular directions, which respectively bisect 
he angles formed by the two optic axes ; the line which bisects 
he smallest angle is called the intermediate section, and the line 
verpendicular thereto which bisects the supplementary angle is 
-alled the supplementary section, These definitions being premised, 
a film of selenite is placed on the diaphragm, with its inter- 
nediate section to the left of the plane of reflection, the successive 
»olarisation is direct or right-handed ; if, on the contrary, it is 
laced to the right of that plane, the successive polarisation is 
ieft-handed. The ray polarised in the intermediate section is 
herefore the most retarded ; and as that section is considered to 
be equivalent to a single optic axis, the crystal is positive. 
In one kind of mica the optic axes are in a plane perpendicular 
to the laminze. They are inclined 223° on each side the perpen- 
dicular within the crystal, but owing to the refraction, are seen 
respectively at an angle of 35° °3 therefrom. 
The principal section is that which contains the two optic axes, 
If the film is placed on the diaphragm with its principal section 
inclined 45° to the left of the plane of reflection, the successive 
polarisation is right-handed, ‘The ray therefore polarised in the 
section which contains the optic axes is the one transmitted with 
the greatest velocity. 
Films of uniaxal crystals, whether positive or negative, andfof 
biaxal crystals, all agree therefore in this respect ; that if the 
plane of polarisation of the quickest ray is to the left of the plane 

