VISIBILITY OF STAR8. 63 



eye, on the rainless shores of Cumana, and on the pla- 

 teaux of the Cordilleras, which are elevated nearly 13000 

 feet above the level of the sea, while I have seen it less 

 frequently and less distinctly in Europe and in the dry 



communicated to the retina by strong light may extend 

 beyond the points excited by it ; and that this secondary 

 vibration may in some degree absorb and neutralise that 

 arising from the second feeble light. Without, however, 

 entering upon these physiological considerations, there is a 

 direct cause to which we may refer the disappearance of 

 the feeble light : viz. , that the rays emanating from the 

 strong light, after forming a perfect image on the retina, 

 are dispersed over all parts of this organ in consequence 

 of the imperfect transparency of the cornea. The rays of 

 the more brilliant body a, in passing the cornea, are affected 

 in the same manner as if they were transmitted through a 

 body whose surface was not perfectly smooth. Some of 

 these regularly refracted rays form the image o, whilst the 

 remainder of the dispersed rays illumine the whole of the 

 retina. On this luminous ground the image of the neigh- 

 bouring object b is projected. This last image must there- 

 fore either wholly disappear or be dimmed. By day two 

 causes contribute to weaken the light of the stars ; one 'is the 

 distinct image of that portion of the atmosphere included in 

 the direction of the star (the aerial field interposed between 

 the eye and the star), and on which the image of the star is 

 formed, while the other is the light diffused by the dispersion 

 which the defects of the cornea impress on the rays emanat- 

 ing from all points of the visible atmosphere. At night, the 

 strata of air interposed between the eye and the star to which 

 we direct the instrument, exert no disturbing action ; each star 

 in the firmament forms a more perfect image, but a portion of 

 the light of the stars is dispersed in consequence of the im- 

 perfect transparency of the cornea. The same reasoning 

 applies to a second, a third, or a thousandth star. The retina 

 then is entirely illumined by a diffused light, proportionate to 

 the number of the stars and to their brilliancy. Hence we 

 may imagine that the aggregate of this diffused light must 

 either weaken, or entirely obliterate the image of the star 

 towards which the eye is directed.'' 



