TISION. 69 



ing to the splendid investigations of Sir William Herschel, 15 

 these diameters decrease with the increasing power of the in- 

 strument. This distinguished observer estimated that, at the 

 excessive magnifying power of 6500, the apparent diameter 

 of Vega Lyrae still amounted to 0"36. In terrestrial objects 

 the form, no less than the mode, of illumination, determines 

 the magnitude of the smallest angle of vision for the naked 



consider a group of stars of the 7th magnitude so close to 

 one another that the intervals between them necessarily escape 

 the eye. If the sight were very clear , and the image of each 

 star small and well defined, the observer would perceive a field 

 of light, each point of which would be equal to the concen- 

 trated brightness of a star of the 7th magnitude. The concen- 

 trated light of a star of the 7th magnitude is sufficient to be 

 seen by the naked eye. The group, therefore, would be visible 

 to the naked eye. Let us now dilate the image of each star of 

 the group on the retina, and substitute a small circle for each 

 point of the former general image; these circles will impinge 

 upon one another, and the different points of the retina will 

 be illumined by light emanating simultaneously from many 

 stars. A slight consideration will show, that, excepting at 

 the margins of the general image, the luminous air has, in 

 consequence of the superposition of the circles, the same 

 degree of intensity as in those cases where each star illu- 

 mines only one single point of the retina; but if each of 

 these points be illumined by a light equal in intensity to the 

 concentrated light of a star of the 7th magnitude, it is evi- 

 dent that the dilatation of the individual images of contiguous 

 stars cannot prevent the visibility of the whole. Telescopic 

 instruments have the defect, although in a much less degree, 

 of giving the stars a sensible and spurious diameter. We 

 therefore perceive with instruments, no less than with the 

 naked eye, groups of stars, inferior in intensity to those which 

 the same telescopic or natural sight would recognize, if they 

 were isolated." Arago, in the Annuaire du Bureau des Longi* 

 tudes pour I' an 1842, p. 284. 



w Sir William Herschel, in the Philos. Transact, for 1803, 

 vol. 93, p. 225, and for 1805, vol. 94, p. 184. Compare also 

 Arago, in the Annuaire pour 1842, pp. 360-374. 



