THE STELLAR UNIYEKSE. 



wonderful results as in its application to the analysis of the starry 

 heavens. 



Two of the chief conditions necessary to distinct vision are, 

 first, that the image on the retina shall have sufficient magnitude : 

 or, what is equivalent to this, that the object or its image shall 

 subtend at the eye a visual angle of sufficient magnitude ; and, 

 secondly, it must be sufficiently illuminated. "When, by reason 

 of their distance from the observer, visible objects fail to fulfil 

 either or both of these conditions, the telescope is capable of 

 re-establishing them. It augments the visual angle by substi- 

 tuting for the distant object, which the observer cannot approach, 

 an optical image of it close to his eye, which he can approach ; 

 and it augments the illumination by collecting, on each point of 

 such image, as many rays as can enter the aperture of the object 

 glass, instead of the more limited number which can enter the 

 pupil of the naked eye ; allowance, nevertheless, being made for 

 the light lost by reflection from the surfaces of the lenses, and by 

 the imperfect transparency of their material. 



37. Although no telescope hitherto constructed has ever pre- 

 sented to an observer the optical image of a star, so as to be seen 

 with any sensible visual angle, such image always appearing as a 

 mere lucid point, it is capable, nevertheless, of increasing the 

 brilliancy of that point in an immense proportion. The way in 

 which it accomplishes this, is easily explained. If the eye be 

 imagined to be directed to a star, as shown in fig. 2, the 

 number of rays diverging from that star, and consequently its 

 apparent brightness, will be limited by, and proportional to the 

 magnitude of the pupil. But if the pencil of rays, before arriving 

 at the eye, be received upon the object glass of a refracting 

 telescope, as shown in fig. 3, or upon the concave reflector of 

 a reflecting telescope, as shown in fig. 4, they will be made to 

 converge to a point, and by proper expedients, the eye being 

 placed near that point, instead of receiving only so many rays, as 

 are proportionate to the dimensions of the pupil, will receive all, or 

 nearly all the rays which pass through the object glass, or which 

 are reflected from the concave speculum. Thus, the intensity 

 of the light received from the object will, by such an instrument, 

 be augmented very nearly in the proportion of the square of the 

 diameter of the pupil to the square of the diameter of the object 

 glass or speculum. Taking, for example, the diameter of the 

 object-glass at 12 inches ; and that of the pupil a little less than 

 the eighth of an inch, the former will be about 100 times the 

 latter ; and it will consequently augment the light received by 

 the eye, in the proportion of about 10000 to 1. 



192 



