Figure 4.10e describes the Hemispherical Cavity.

The cavity is created by one plan mirror, and one spherical mirror with radius of curvature equal to the length of the cavity.
 

This cavity is similar in properties to circular optical cavity, with the advantage of the low price of the plan mirror.

Most Helium-Neon lasers use this cavity which have low diffraction losses, and is relatively easy to align.

Advantages:

• Low sensitivity to misalignment.
• Low diffraction losses.