CIRCULAR POLARIZATION OF LIGHT. $5 
through a plate of definite thickness that we can determine, whether the 
Qn—1 ing Qn+1 
4 4 
If, however, the same plate is examined in the different parts of the 
spectrum, we obtain by the experiments just mentioned x itself. It is 
manifest that if we wish to obtain by refraction phenomena of circular 
polarization in white light, it is advisable so to determine the thickness 
of the plate or the temperature of the glass that the difference of path 
for the central rays will become 4 undulation. For this purpose I use 
the flame of alcohol coloured yellow by common salt or nitrate of soda. 
undulation. 
difference of path of the two rays be 
5. Phenomena of Colours of combined Crystals in White Light. 
It now becomes easy to account for the complicated phenomena of 
colours obtained by the insertion of a crystallized plate parallel to the axis 
and of any given thickness behind a crystallized lamina cut perpendicular- 
ly to the axis. For as the light is circularly polarized for one colour on 
the right, for the other on the left, and rectilinearly for an intermediate 
one, the black tufts on their two sides assume different colours: the 
phznomena -in the even quadrants differ essentially from. those in the 
odd ones, but the rings of colours in both are essentially different from 
the succession of colours in Newton’s rings. The phenomenon may be 
previously determined from the known values of the indices of refraction, 
the length of waves for the homogeneous rays of the spectrum, and the 
thickness of the plate; but it may also be experimentally exhibited by 
adjusting the condensing-lens p of the apparatus so that the spectrum 
in the aperture of the Nicol’s polarizing prism e be concentrated to white; 
a confirmation, the frequent repetition of which, howeyer, is not advis- 
able, on account of the intensity of the light of the apparatus. 
6. Phenomena of Colours in Twin- Crystals. 
Tn passing from the artificial combinations of two crystals to natural 
twin-crystals we have to distinguish them into three classes: namely, 
the axes of the united individual crystals are either perpendicular or pa- 
rallel to each other, or they are inclined at some angle with one another. 
The section is always to be made perpendicular to the axis of one of 
. the individual crystals. Though the first case may immediately give 
the phenomena just mentioned, yet, as far as I am aware, it does not 
occur with transparent crystals, whilst the second case may occasion the 
phenomena of colours with biaxal crystals only. Thus, if (as for in- 
its erystallographicvaxis lies parallel to the axis of the crystal which is 
divided by it into two parts, these two parts (since the optical axes of 
this lamella render perceptible, however small, angles with the bounding 
planes, ) will operate as double-refracting prisms upon the light passing 
through these axes, because their optical axes do not lie in the plane of 
G2 
