98 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 30 Ser. 



conditions of clearness and serenity, the apparently ap- 

 proaching image of the star is a beautiful, steady, minute 

 disc with one or more delicate diffraction rings. The 

 Moon's white and sharply defined border exhibits all its 

 serrations and irregularities. As it approaches the star, 

 the diffraction rings disappear in the increasing light in the 

 field of view, but the disc remains; and at their visible con- 

 tact the disappearance of even the largest and brightest star 

 is absolutely instantaneous; and no observer can be in 

 doubt of the epoch beyond the tenth of a second. Under 

 such favorable atmospheric conditions, the image of the star 

 never enters upon the visible limb of the Moon, no matter 

 what the size of the objective or the magnifying power 

 employed. And with such conditions, the reappearance of 

 a star from the bright limb of the Moon will be instan- 

 taneous. If the star were apparently to make a near 

 approach to the Moon along the dark north or south limb, 

 where there are serrations, it might pass so close as to dis- 

 appear behind the first mountain and reappear in the next 

 valley (each phenomenon being absolutely instantaneous) to 

 be swallowed up by the next mountain, or in its absence, to 

 continue its visible course. We have heard (1846) the 

 elder Bond describe such a phenomenon in his experience 

 and it has happened once to ourselves. But when the 

 atmosphere begins to change to unsteadiness, the star loses 

 its diffraction rings, the nucleus is broken up and gradu- 

 ally diffused into a nebulous image, fuzzy and unsteady, 

 with a brighter, irregular, dancing nucleus, or is spread out 

 as a nebulous film as much as forty or fifty seconds of arc 

 in diameter. This unsteadiness of the atmosphere throws 

 the disc of the Moon into irregular vibrations or displace- 

 ments of equal amplitude and duration, giving it a factitious 

 border. With this disturbed and spurious limb the Moon 

 approaches the diffused image of the star, both being in a 

 state of great unsteadiness; but if the nucleus of the star 

 be sufficiently large, bright, and colored, like Antares and 

 Aldebaran, the impression of its image upon the retina of 

 the eye is naturally more intense as an isolated spot than 



