Similarly, drops transmitting rays after two internal reflections will be situated on covertical and **coaxial** cones, of which the semi-vertical angles are 51° for the red rays and 54° for the violet.

Capacity of two concentric spheres, of two **coaxial** cylinders and of two parallel planes.

The same arrangement can be supplied to a pair of **coaxial** cylinders.

The other case of importance is that of two **coaxial** cylinders.

Let a solid circular sectioned cylinder of radius R 1 be enclosed in a **coaxial** tube of inner radius R2.

The isothermal surfaces are **coaxial** cylinders.