through a plate of definite thickness that we can determine, whether the difference of path of the two rays be or or undulation. If, however, the same plate is examined in the different parts of the spectrum, we obtain by the experiments just mentioned itself. It is manifest that if we wish to obtain by refraction phænomena 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 ¼ undulation. For this purpose I use the flame of alcohol coloured yellow by common salt or nitrate of soda.
5. Phænomena of Colours of combined Crystals in White Light.
It now becomes easy to account for the complicated phænomena of colours obtained by the insertion of a crystallized plate parallel to the axis and of any given thickness behind a crystallized lamina cut perpendicularly 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 phænomena 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 phænomenon 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 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, however, is not advisable, on account of the intensity of the light of the apparatus.
6. Phænomena of Colours in Twin-Crystals.
In 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 parallel 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 phænomena just mentioned, yet, as far as I am aware, it does not occur with transparent crystals, whilst the second case may occasion the phænomena of colours with biaxal crystals only. Thus, if (as for instance in arragonite,) a very thin crystal is so united with another that its crystallographic axis 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
