optical effects

At first they seemed to be optical effects, as though the eye itself were playing tricks.

These different optical effects are classed together under the name of anthelia (from Greek, opposite to, and Greek, the sun).

Very different optical effects, however, follow if the rays from the various-coloured flames are made to pass through a prism.

Hitherto it had been supposed that double stars were mere optical effects.

There are other optical effects to be seen sometimes on the horizon somewhat resembling twilight.