OCCULTATION OF JUPITER, 1857. 443 



instrument is by Ross, the object-glasses 3f inches diameter, 

 focal length 5 1 inches.; portions (about j inch) of the glasses 

 are ground away to enable them to be approximated suffi- 

 ciently for the distance between the eyes of the observer. The 

 power used was 60, which gave a greater idea of magnitude 

 than double that power in a monocular telescope. 



The dark limb of the moon was faintly visible throughout 

 the time of the occultation. The disappearance of the satel- 

 lites was not instantaneous, as with a fixed star, but (as indeed 

 might have been expected) they gradually faded away. The 

 light of the moon's edge was too feeble for me to ascertain 

 whether they disappeared at the exact edge or not. I should 

 say that they seemed to be a little within it, but I could not 

 positively assert this. Jupiter was cut off with perfect sharp- 

 ness, and I could not detect the slightest distortion. When 

 about one-third was eclipsed the light of the planet appeared 

 brighter along the edge of the moon, and this apparently in- 

 creased brightness continued until it was about three-fourths 

 eclipsed. Two spots in the neighbourhood of the upper belt 

 (as viewed by inverting telescope) were notably bright, so as 

 to attract attention, and they continued so during the period 

 above-mentioned, each looking somewhat like an electric glow 

 coming off the dark edge of the moon. This effect may be 

 optical, and caused by the immediate proximity of darkness 

 to central and more luminous portions of the planet, but it 

 was very marked. I observed no other effect during the 

 period of immersion. I looked for an effect said to have been 

 observed on previous occasions, viz. a faint illumination of the 

 dark edge of the moon upon contact, but could not detect the 

 slightest trace of it. On emersion I could detect no distortion 

 of any sort, nor did I see the dark line which I noticed in the 

 occultation of Nov. 8, 1856. The difference of light and 

 colour between the planet and the moon was the same as then 

 observed. 



After emersion there was a decided stereoscopic effect, 

 seen both by Mr. Gillett (who up to this time had left the 

 telescope entirely to me) and myself ; Jupiter in the binocular 

 appearing as at a distance behind the moon. Mr. Gillett tells 

 me that with terrestrial objects the effect of the binocular, if 



