The phenomenon of colour may, however, be obtained with thick plates by superposing two of them in a suitable manner, the combination acting as a thicker or a thinner plate according as the planes of polarization of the quicker waves within them are parallel or crossed.
Towards the end of the summer the uredospores are replaced by the winter resting-spores, called teleutospores, which are larger, thicker-walled and darker in colour.
In "capped quartz" these layers are thicker, and the successive shells of the crystal may be easily separated.
The majority of the members of the family are of longer duration, and have the roots also fibrous, but fewer, thicker and less branched.
Hence the tension is the same for all films thicker than e, the range of the molecular forces.