The Camera Lucida, sometimes known as Wollaston prism, consists of an upright brass column, to which a prism holder is mounted. By looking through the prism you can see a sheet of paper on the table in front of you and superimposed an image of an object which you want to sketch on the paper.
The Camera Lucida facilitates to make a drawing of a microscopic object (using a suitable objective lens!) or of a macroscopic object.
further reading
Fiorentini, Erna (2006): Camera Obscura vs. Camera Lucida – Distinguishing Early Nineteenth Century Modes of Seeing. MPI für Wissenschaftsgeschichte PREPRINT 307 (full text as PDF)
Geimer, Peter (2001): Photographie und was sie nicht gewesen ist - Photogenic Draws 1834-1844, in: Dürbeck, Gabriele, u.a. (Hrsg.): Wahrnehmung der Natur, Natur der Wahrnehmung - Studien zur Geschichte visueller Kultur um 1800, Dresden: Verlag der Kunst.