(n)optical device consisting of a tube containing a convex achromatic lens at one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam
The Collaborative International Dictionary of English (GCIDE) v.0.53Collaborative International Dictionary (GCIDE)
n. 1. (Astron.) A telescope arranged and used to determine errors of collimation, both vertical and horizontal. Nichol. [ 1913 Webster ]
2. (Optics) A tube having a convex lens at one end and at the other a small opening or slit which is at the principal focus of the lens, used for producing a beam of parallel rays; also, a lens so used. [ 1913 Webster ]