ClipArt ETC
"...a tube with a slit at the further end through which the light enters, and at the other end a collimating lens which brings the rays into a parallel beam (the slit is formed between two parallel edges the distance between which can be varied at will)..." —Whitney, 1889

Prism Spectroscope from the Late 19th Century

"...a tube with a slit at the further end through which the light enters, and at the other end a collimating…