TELESCOPES. 



95 



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

 of Jupiter with one of Dolland's telescopes, of a magnifying 

 power of only 95, when the sun was already from 15 to 18 

 above the horizon. The diminished intensity of the light 

 of Jupiter and Saturn, when seen by day in the great Berlin 

 refractor, especially when contrasted with the equally reflected 

 light of the inferior planets, Venus and Mercury, frequently 

 excited the astonishment of Dr. Galle. Jupiter's occul- 



; 



" L' experience a montre que pour le commun des homines., 

 deux espaces eclaires et contigus ne se distingueiit pas Pun 

 de 1'autre, a moins que leurs intensites comparatives ne pre- 

 sentent, au minimum, une difference de -^. Quand une lu- 

 nette est tournee vers le firmament, son champ semble uni- 

 formement eclaire : c'est qu' alors il existe, dans un plan 

 passant par le foyer et perpendiculaire a 1'axe de I'objectif, une 

 image indefrnie de la region atmospherique vers laquelle la 

 lunette est dirigee. Supposons qu'un astre, c'est-a-dire un objet 

 situe bien au-dela de F atmosphere, se trouve dans la direction de 

 la lunette : son image ne sera visible qu'autant qu'elle augmen- 

 tera de -g^-, au moins, 1'intensite de la portion de 1 image 

 focale indtfinie de 1' atmosphere, sur laquelle sa propre image 

 limitee ira se placer. Sans cela, le champ visuel coiitinuera a 

 paraitre partout de la meme intensite." 



" Experience has shown that, in ordinary vision, two illu- 

 minated and contiguous spaces cannot be distinguished from 

 each other, unless their comparative intensities present a mini- 

 mum difference of -g^th. When a telescope is directed towards 

 the heavens, its field of view appears uniformly illumined : 

 there then exists in a plane passing through the focus, and 

 perpendicular to the axis of the object-glass, an indefinite 

 image of the atmospheric region towards which the instru- 

 ment is pointed. If we suppose a star, that is to say, an object 

 very far beyond the atmosphere, situated in the direction 

 of the telescope, its image will not be visible, except it exceed, 

 by at least -g^-th, the intensity of that portion of the indefinite 

 focal image of the atmosphere on which its limited proper 

 image is thrown. Otherwise, the visual field will contimie to 

 appear everywhere of the same intensity." 



