72 COSMOS, 



while, in accordance with another law, they mfluence the 

 aerial space on which the fixed star is projected. The tele- 

 scope, by separating, as it were, the illuminated particles of 

 air surrounding the object-glass, darkens the field of view, 

 and diminishes the intensity of its illumination. We are en- 

 abled to see, however, only by means of the difierence be- 

 tween the light of the fixed star and of the aerial field or the 

 mass of air which surrounds the star in the telescope. Plan- 

 etary disks present very different relations from the simple 

 ray of the image of a fixed star ; since, like the aerial field 

 {Vair aerienne), they lose in intensity of light by dilatation 

 in the magnifying telescope. It must be further observed, 

 that the apparent motion of the fixed star, as well as of the 

 planetary disk, is increased by high magnifying powers. 

 This circumstance may facilitate the recognition of objects 

 by day, in instruments whose movements are not regulated 

 paralactically by clock-work, so as to follow the diurnal mo- 

 tion of the heavens. Difierent points of the retina are suc- 

 cessively excited. " Very faint shadows are not observed," 

 Arago elsewhere remarks, " until we can give them motion." 

 In the cloudless sky of the tropics, during the driest sea- 

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

 disk of Jupiter with one of Dollond's telescopes, of a magni- 

 fying 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, especiallj^ when contrasted with the equally 

 reflected light of the inferior planets, Venus and Mercury, 

 frequently excited the astonishment of Dr. Galle. Jupiter's 



deld de I'atmosphere, se trouve dans la direction de la lunette : sou 

 image ne sera visible cju'autant qu'elle augmentera de -^q, au raoins, 

 I'iutensite de la portion de I'image focale ind^Jinie de I'atmosphere, sur 

 laquelle sa propre image limil^e ira se placer. Sans cela le champ 

 visuel continuera Si paraitre partout de la meme intensite." 



" Experience has shown that, in ordinary vision, two illuminated and 

 contiguous spaces can not be distinguished from each other unless their 

 comparative intensities present a minimum difference of ^'fj^th. When 

 a telescope is directed toward 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 im- 

 age of the atmospheric region towai'd which the instrument is pointed. 

 If we suppose a star, that is to say, an object very far beyond the atmos- 

 phere, situated in the direction of the telescope, its image will not be 

 visible except it exceed, by at least ^f^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 contirue to 

 appear esery where of the same intensity. 



