MADE AT LANCASTER. 63 



25th, Immersion of the 1st satellite of Jupiter, observed at 

 9 h 5' 7" mean time, or 8" 52' 19" apparent time. 



Emersion, of the 4-th satellite of Jupiter, observed at ll h 32' 



42" mean time, or ll 1 " 19' 5 3" apparent time. The night 

 was remarkably line, — the belts and satellites perfectly defined : 

 — magnifying power 100.- 



February 6th, Immersion of the 2d satellite of Jupiter, ob- 

 served at 9 k 51' 4G" mean time, or 9 h 37' 15" apparent time: — 

 The night a little hazy: — magnifying power 100. 



9th, The moon occulted a number of stars in the northern 

 part of that cluster called the Pleiades: the occupations of 

 the three largest were particularly attended to.- — The immcr- 

 sons were all instantaneous; but the emersions could not be ob- 

 served on account of the houses on the west side of the street. 

 Not having either the places, or characters of the stars obser- 

 ved, I have in Fig. 3d, Plate III, laid down the relative posi- 

 tions of the principal ones in the cluster, as nearly as I could 

 do it without the aid of any other instrument than my telescope., 

 and numbered those that weie occulted. 



h i a h i a 



No. 1 immersed »t 10 23 54} 10 9 16} 



2 - - do. - - 10 31 38 v. mean time, or at 10 17 C apparent time. 



3 - - do. - - 10 S3 25 10 38 24} 



In the diagram, ABCE represents the dark part of the 

 moon's disk, and A E C D the enlightened part. When the 

 enlightened part was kept out of the field of the telescope, 

 the limb of the dark part was sufficiently visible, and well 

 defined; by which I was enabled, without fatiguing my eye, 

 to trace the approach of the moon to the stars, and pay that 

 close attention so very necessary at the instant of the occulta- 

 tion. The star No. 1 is very small, and seldom visible with- 

 out the aid of a glass. In each case the star appeared for a 

 few seconds well defined on the edge of the moon's disk. — 

 Various theories have been devised to account for this singular 

 phenomenon, but I am inclined to believe with La Lande, that 

 it is merely an optical illusion*. 



* 11 arrive souvent dans les eclipses d' etoiles ou de pianettes par la lune, que 1'astre eclipse paroit 

 tout entier pendant quelques secondes sur le disque eclaire de la lune ; on a attribue ce phenomene 

 a l'atmosphere de la lune, et M. Euler enterprend de prouver son existence par les tclips&s de solcil 



