E. Cobham Brewer 1810–1897. 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 painter’s brush made of the hair of a cow’s tail); so called because they are like the hairs of a paint-brush, except at the point where they aggregate.
