94 COSMOS. 



facilitate the recognition of objects by day, in instrument 

 whose movements are not regulated paralactically by clock- 

 work, so as to follow the diurnal motion of the heavens. 

 Different points of the retina are successively 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 season 



which it is seen, will be the same to the naked eye and in 

 telescopes, whatever may be their dimensions and magnifying 

 powers. Telescopes, therefore, do not favour the visibility of 

 planets in respect to the intensity of their light. 



The same is not the case with respect to the stars. The 

 intensity of the image of a star is greater when seen with 

 the telescope than with the naked eye ; the field of vision, 

 on the contrary, uniformly illumined in both cases by the 

 atmospheric light, is clearer in natural than in telescopic 

 vision. There are two reasons then, which, in connexion 

 with the consideration of the intensity of light, explain why 

 the image of a star preponderates in a telescope rather than 

 in the naked eye over that of the atmosphere. 



"This predominance must gradually increase with the 

 increased magnifying power. In fact, deducting the constant 

 augmentation of the star's diameter, consequent upon the 

 different effects of diffraction or interference, and deducting 

 also the stronger reflection experienced by the light on the 

 more oblique surfaces of ocular glasses of short focal lengths, 

 the intensity of the light of the star is constant, as long as the 

 aperture of the object-glass does not vary. As we have 

 already seen, the brightness of the field of view, on the con- 

 trary, diminishes incessantly in the same ratio in which the 

 magnifying power increases. All other circumstances, there- 

 fore, being equal, a star will be more or less visible, and its 

 prominence on the field of the telescope will be more or less 

 marked, in proportion to the magnifying powers we employ.'* 

 Arago, Manuscript of 1847. 



I will further add the following passage from the Annuaire 

 du Bureau des Lwig. pour 1846 (Notices Scient. par M. Arago} 

 p. 381. 



