346 CALIFORNIA ACADEMY OF SCIENCES. 



TRANSIT OF JUPITER'S IV SATELLITE, Sunday, June 7, 1885. 



[Abstract of note by Prof. George Davidson. The observations were made 



at the Davidson Observatory, with the 6.4 inches Equatorial; 



objective by Alvan Clark & Sous.] 



Just after sunset the telescope was pointed to Jupiter, 

 and instantly my eye caught the small black spot on Jupiter's 

 disk near the apparent eastern edge, and going off. In the 

 bright twilight the belt of Jupiter was not strongly marked, 

 but when more attention was given to it the black spot was 

 seen traversing along the apparent lower edge of the belt 

 where it was relatively dark; it was probably one-fourth of 

 its diameter on the belt; the area of the other three-fourths 

 was on the bright body of the planet. The image of the 

 planet was very much disturbed by the unsteadiness of our 

 atmosphere, and it would not bear a power of 300 or 250 

 diameters. A power of 120 diameters gave the best results. 

 The local mean time of the first observation was 7h. 28m. 

 The satellite was watched continuously, and presented the 

 same black image; and at momentary intervals it was sharply 

 defined. Occasionally I think it is elongated in the line of 

 the belt (7h. 40m. to 7h. 50m.), but I cannot be positive of 

 its persistence, because such an appearance might be due to 

 the atmospheric waves of disturbance. At 7h. 50m. the 

 belt appeared darker in the decreasing twilight and the sat- 

 ellite jet black, approaching the planet's limb. At 8h. 00m. 

 it was difficult to make out the satellite even when the at- 

 mosphere was somewhat steady for a moment or two. But 

 three minutes later the image was a very black sharp point. 

 The atmosphere was quiet for a short time and there was no 

 doubt about its color. The image soon began to lose its 

 color, nor did the belt appear so dark as before. When the 

 satellite was about one diameter from the planet's limb (at 

 8h. 19m.) it was very difficult to detect the image even at the 

 best moments; but when it had approached within half a 



