ON THE APPARENT FIGURE OF STARS, 



57 



star has the appearance of a circular disk^ surrounded by a co- 

 loured aureola. 



The coloured aureola, that accompanies the circular image of Coloured au- 

 the stars, when their light is too weak for the rays to be per- ^^^^.^ round a 

 ceived, is independent of the cause that produces this latter 

 phasnomenon. It appears to be occasioned by the object glass 

 being more or less imperfectly achromatic ; by the faintness of 

 the image, which permits the aureola to be distinguished ; by 

 the inflection of the ray of light at the edges of the diaphragm, * 



that lessens the object glass ) and by several other causes, on 

 which I shall not attempt to enlarge in this paper. 



As the rays that accompany luminous bodies are not per- Disc of a pla- 

 ceived, iJnless the object producing them be seen under a very net magnified, 

 small angle, it follows, that when the diameter of a planet is 

 increased by the assistance of a telescope, so as to be seen under 

 an angle of some minutes, the rays disappear, and the planet is 

 clear and well defined. 



In these rays there is this particularity, that their direction direction of 

 always depends on that of the eyes looking at the object, the rays de- 

 Thus, if, when looking at a distant light, we incline the head, ^y"*^*"" ^^^ 

 as at fig. '] , we shall immediately perceive the direction of the 

 rays change. One of these directions is constantly parallel with 

 the two eyes, moving with them ; and the others preserve their 

 relative situation with respect to this. 



The rays distinguished round luminous objects may be pro- The rays pro- 

 duced, either by the luminous body itself, or by the organ that <i,'iced by the 

 discerns them. In the first case, the number and position of ^ 



the rays should be the same to every spectator : but, as the 

 number of these rays varies to different eyes, and follows th6 

 direction of the eyes when these are inclined in looking at the 

 object, it follows, that the rays are produced by the organ per- 

 ceiving them. ^ 



This truth is farther confirmed by looking at stellate lights They are de- 

 through a small aperture 3 as this immediately destroys the rays, ^\^^Y\n ^ 

 and the luminous bodies appear of smaller dimensions. In this through a 

 case their figure is altered only by the inflection of the light small hole, 

 at the edges of the small aperture j an inflection which gene- 

 rates aureola round the images of luminous objects. 



Since the rays, that appear to emanate from luminous objects How does th» 

 seen at a great distance, are produced by the eye that perceives ^y^ produce 



Vol. XXXIII, No. 152.— October, 1812. H them j 



