We seem in looking at the lunar halo, then, to see the moon at the centre of a dark space, surrounded by a ring of bright particles, outside which again are particles not quite so brightly illuminated as those forming the ring, but more brightly than those within the ring. But in reality this impression, which, so far as the sense of sight is concerned, seems _forced_ upon the mind, is entirely erroneous. There is no real distinction between the space which looks dark all round the moon, the space beyond which does not look dark, and the ring between the two spaces which looks bright. These are all equally illuminated by the moon, in the same sense, at least, that we say the surface of a moonlit sea is all equally illuminated, neglecting slight differences which do not concern the point we are specially dealing with. Precisely as the path of light on the ocean is not a real path of illumination, bounded on either side by dark spaces, so the ring of light round the moon is not a real ring of light, bounded on one side by a less bright region, and within by a dark space. [Illustration: Fig. 11.--Lunar Halo] Although my object in these essays is not specially to deal with scientific matters, but rather with the thoughts (much more important in my belief) which they suggest--so that, in dealing with my present subject, I wish rather to call attention to the manifold ways in which our senses may deceive us unless their evidence is carefully cross-examined--yet it may be worth while to notice how the particular illusion here considered has deceived even the scientific elect.