108 Edward Sang, Esq., on the 



separate roundish beads began to appear ; these became more 

 frequent as the angle of intersection grew less. 



On attending closely to that part of the moon's edge, 

 which was about to pass off the sun's disc, whenever we 

 found an indentation and followed it up to the cusp, the hol- 

 low was seen to separate from the rest of the illuminated 

 surface, and to assume a roundish form, which fluctuated 

 with the undulations. Carefully observed, it seemed that 

 the indistinct roundness of the form was due to the retention, 

 by the retina, of the rapidly succeeding impressions. 



At the incoming cusp again, whenever a bead made its 

 appearance and was followed until it united with the rest of 

 the illuminated surface, that portion of the moon's limb which 

 had come on with it was found to be deeply indented. The 

 last of this class of phenomena which we noticed was so dis- 

 tinct, as of itself to be decisive. A large and well separated 

 bead was seen coming on, and its distance from some irre- 

 gularities on the moon's edge was estimated. After the 

 bead had amalgamated, that part of the moon's edge which 

 had the same distance from these irregularities was found to 

 be marked by two prominences whose height above the 

 general surface might be 3" or 4", and whose summits were 

 at the distance of 8", with a deep hollow of about 6" between 

 them. 



On the whole, the impression on my mind was confirmatory 

 of my previous opinion, that these detached portions of light 

 result simply from the ruggedness of the moon's edge. I 

 had not previously had an opportunity of witnessing the 

 appearance under such favourable circumstances. What I 

 have now seen has convinced me that the distortions arising 

 from the unequally heated state of the atmosphere, or from 

 the imperfections of the telescope, have been taken for some- 

 thing else. 



The decision of this question is not without important 

 results in practical astronomy, for the instant of occultation 

 of a star must be subject to an uncertainty depending on the 

 configuration of that part of the moon's limb on which it 

 falls ; and this uncertainty must pervade all determinations 



