Illustration of a man looking through a spectroscope.
From the corresponding text: "Fig. 52, especially the three tubes directed to the prism at different angles, as in that constructed by Kirchhoff and Bunsen. The eye of the observer is placed in the axis of the telescope directed to that surface of the prism from which the light emerges in the form of the spectrum; the opposite surface of the prism receives through the slit and collimating lens the light emitted from the object to be examined; at the side of the observer is the tube carrying the illuminated scale, or the micrometer screw, so that the mark coinciding with any division of the scale may be placed on any line of the spectrum."