The perforated ring dial is made in the form of a rather broad circular band pierced in the middle of one side with a tiny hole through which a sunbeam was projected among hour numbers inscribed on the opposite interior surface of the ring. The hour numbers are usually too crowded for accurate reading unless the ring is very large, in which case they lose their portability.
Figure 57 shows two perforated ring dials. They can be used at any time of the year by sliding the tiny hole to the correct date. They are designed for use at a certain latitude. The tiny hole is moved such that it points toward the Sun so that the sunray will pass through the tiny hole, and hit the hour numbers on the interior surface of the ring. The hour number that the sunray hits gives the time.