Note 26 — Chapter III
Reading edition
This centring-glass consists of a tubular cap with a minute aperture, containing two plano-convex lenses, so adjusted that the image of the aperture in the object-glass and the images of the aperture of the lenses and the diaphragms contained in the tube which holds the illuminating combination, may be all in focus at the same time, so that by the same adjustment they may be brought sufficiently near to recognise their centricity.