LUNAR CELESTIAL PHENOMENA.

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moon’s semidiameter subtends an angle of less than 16'.Now the sun appears wholly in view when in realityhe is below the level of the horizon, our atmospherehaving sufficient refractive power to raise the sun’simage by about 34' (his diameter is about 3V). Andthis action is produced on rays which have only passedthrough the atmosphere to reach the earth tangen-tially. In passing out again, such rays would bedeflected through 34' more, or in all by about 68'.Accordingly, since 16' is less than a quarter of 68', ifthe moon’s atmosphere possessed only a fourth partof the refractive power of our own atmosphere, a starin reality behind the centre of the moon’s disc wouldappear as a ring of light. Nor would this ring be veryfaint. The light of the star would not be diluted orspread over the ring and therefore reduced in corre-sponding degree. On the contrary, the moon’s atmo-sphere would act the part of an enormous lens, in-creasing the total quantity of light received from thestar, in the same way that the lens of a telescope’sobject-glass increases the quantity of light receivedfrom any celestial object.*

* An effect, indeed, somewhat similar to that here considered,way be produced by covering all but the outer ring of an object-glass with a black disc, and removing the eye-piece ; if then, thetelescope be directed nearly towards a bright star, and shiftedfrom that position until exactly directed on the star, the lightfrom the star will be presented in the form of an arc, graduallyextending farther and farther round until it forms a completecircular ring.

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