66 SPECIAL RESULTS IN THE TJRANOLOGICAL 



air which surrounds it, in the telescope. The case of the 

 simple ray of the image of a fixed star differs from that of 

 planetary discs ; the latter, under the magnifying power of 

 the telescope, losing in intensity of light by dilatation 

 equally with the aerial field from which they detach them- 

 selves. It is also to be noticed that high magnifying 

 powers increase the apparent rapidity of motion in the fixed 

 stars as well as in the planets. In instruments which are 

 not mounted equatorially, and made to follow the movement 

 of the heavens by means of clock-work, this circumstance 

 may facilitate the recognition of objects in the day-time. 

 New points on the retina are successively stimulated. Yery 

 faint shadows, as Arago has remarked elsewhere, first become 

 visible by being put in motion. 



Under the serene sky of the tropics in the driest season 

 of the year, I have often been able to find the pale disc of 

 Jupiter, in a Dollond's telescope, with the low magnifying 

 power of 95, when the sun was already 15 or 18 above; 

 the horizon. The f'aintness of the light of Jupiter and Sa- 

 turn seen in the day-time in the large Berlin refractor, and 

 contrasted with the brighter, though equally reflected, light 

 of the planets nearer to the sun, Mercury and Yenus, has 

 repeatedly surprised Dr. Galle. Occupations of Jupiter's 

 satellites have sometimes been observed in the day-time with 

 powerful telescopes (by Plaugergues, 1792; and Struve, 

 1820). Argelander, at Bonn (7th December, 1849), saw 

 very clearly three of Jupiter's satellites a quarter of an hour 

 after sunrise with a 5 -feet IVaunhofer. He could not 

 recognise the fourth satellite. His assistant, Herr Schmidt, 

 saw still later the emersion from behind the dark limb of 

 the moon of all the satellites including the fourth, in the 



