138 Apparent Visibility of Stars through the Moon. 



of its perfect image when brought to a focus. In the occul- 

 tation of the star Aldebaran in 1829 the reason why eight of 

 the thirty-one European observers did not perceive any pro- 

 jection, and that the other twenty-three did, may be, that 

 the telescopes of the former were suited to the lunar focus, 

 and those of the latter to the stellar ; the eye being incapable 

 of determining the exact focus. 



I did not, however, publish my remarks, nor shew them to 

 any one until last month, when my nephew (Frederick B. 

 Edmonds) being here on a visit, I desired him to read them, 

 with a view to test by experiment the correctness of my ex- 

 planation. He accordingly placed a candle in the furthest 

 corner of the room close behind a card, through a small hole 

 in which the light flowed to represent a star. At the distance 

 of about two yards from the candle he placed an illuminated 

 disc to represent the moon ; and then retiring three yards 

 from the disc, with a powerful pocket spy-glass, having its 

 focus set for the "star," looked at the " star," along the edge 

 of the " moon," when the former appeared very clearly pro- 

 jected on the latter, precisely as in the reality observed by 

 astronomers. When the focus of the glass was set for the 

 "moon" no projection whatever occurred. 



I immediately communicated this to Professor Airy, who 

 very kindly informed me that the explanation would be satis- 

 factory if the focal length of the telescope for the moon were 

 sensibly different from that for the star. " It would be highly 

 desirable, however, (he added,) to bear this consideration in 

 mind in the case of another observation of the occultation of 

 a bright star." 



The explanation now offered will therefore, in all proba- 

 bility, ere long be fully tested ; and if the eye be unable 

 directly to detect any difference between the lunar and stellar 

 foci, the existence of a sensible difference between them 

 would, I presume, be indirectly established, should the pro- 

 jection disappear on lengthening the focal distance, and re- 

 appear on shortening it. 



