E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Pencil of Rays.
All the rays that issue from one point, or that can be focussed at one point (Latin, penicillus, little tail, whence penicillum, a painters brush made of the hair of a cows tail); so called because they are like the hairs of a paint-brush, except at the point where they aggregate.