II. Description of an apparatus employed at the Kew Observatory, Richmond, for the examination of the dark glasses and mirrors of sextants

In the "Proc. Roy. Soc.," vol. 16, p, 2, Professor Balfour Stewart described an apparatus designed and constructed by Mr. T. Cooke for the determination of the errors of graduation of sextants. This instrument has from that date been constantly in use at the Kew Observatory, and since the introduction of certain unimportant improvements, has been found to work very well. No provision was made, however, for its employment in the determination of the errors of the dark shades used to screen the observer’s eyes when the sextant is directed to the sun or moon, and it has been found that errors may exist in the shape of want of parallelism in these glasses, sufficiently large to seriously affect an observation, accurate in other respects.