148 Proceedings of Philosophical Societies. [Feb. 



about half their diameters were immersed, when it disappeared. 

 All the satellites presented this phaenomenon; but the fourth and 

 third with the greatest distinctness. On Jupiter's approach, no 

 difference of his light or shape was perceptible, but after the 

 contact had taken place, he appeared to exhibit no deficiency of 

 disc, but presented a complete figure, as if placed between the 

 moon and the earth, this appearance continuing for a few 

 seconds. When the planet was almost entirely immersed, his 

 retiring limb appeared as though it were considerably elongated, 

 or formed a segment of a much larger circle than had been pre- 

 viously presented. The position of Mr. Ramage's telescope did 

 not allow him to observe the emersion. 



Capt. Ross was prevented by the state of the weather from 

 seeing the immersioit, but was fortunate enough to observe the 

 emersim, seeing first a considerable elongation, which gradually 

 diminished as more of the planet appeared from behind the moon. 



Part of a letter was read from Mr, R. Comfield, a Member of 

 this Society, in reference to the same occultation. He observed 

 it at Northampton with a good Newtonian reflector. Mr. Com- 

 field, and two other contemporaneous observers, with good 

 instruments, noticed that when Jupiter had about half disap- 

 peared, there was exhibited an adhesion or protuberance on each 

 side of the planet, which, as Jupiter sunk behind the moon, be- 

 came larger and larger, so that just before the entire disappear- 

 ance of the planet, it exhibited a considerable elongation deviat- 

 ing greatly from a circular curve of the same diameter as the 

 planet. 



Phenomena, somewhat analogous, especially in reference to 

 the indentations and adhesions, were noticed by several astro- 

 nomers who observed the transit of Venus in 1769. See the 

 account by Capt. Cook, Mr. Charles Green, Mr. Charles Mason, 

 M. Pingre, &c. in the Phil. Trans, for 1770 and 1771, which are 

 here adverted to, because the consideration of kindred pheeno- 

 mena may assist in the explication of the whole. 



Jau. 14, 1825. — At the meeting this evening, Mr. Baily laid 

 on the table for the inspection of the members, two micrometers, 

 which have been recently invented and constructed by M, 

 Frauenhofer of Munich. 



These micrometers are formed by means of very fine lines, cut 

 on glass with a diamond point in a peculiar manner, and placed 

 in the focus of the telescope. One of these micrometers consists 

 of concentric circular lines drawn at unequal distances from each 

 other ; and the other consists of straight lines crossing each 

 other at a given angle. The mode of cutting these lines has 

 furnished M. Frauenhofer with a method of illuminating them, 

 which (at the same time that it renders the lines visible) leaves 

 the other part of the field of the telescope in darkness ; so that 

 the transits of the smallest stars may be observed by means of 

 these micrometers ; the hnes appearing like so many silver 



