50 SPECIAL IlESULTS IN THE ITRANOLOGICAL 



satellites have at times., as Arago states, ( 105 ) an intenser 

 light, on equal surfaces, than the planet : at times, on the 

 other hand, they appear on the face of Jupiter, as recent 

 observations inform us, as gray spots. 



The rays, which appear to our eyes to issue from the planets 

 and from the fixed stars, and which from the earliest times 

 have been employed in pictorial representations, particularly 

 among the Egyptians, to designate the shining heavenly 

 bodies, have a length of at least five or six minutes. Has- 

 senfratz declares them to be focal lines, " intersections de 

 deux caustiques," on the crystalline lens. " The image of 

 stars seen by us with the naked eye is enlarged by diverging 

 rays : by reason of this extension it occupies a larger space 

 on the retina than if it were concentrated in a single point. 

 The impression on the nerve is weaker. A very dense clus- 

 ter of stars, in which all the single stars hardly attain the 

 7th magnitude, may, on the other hand, become visible to 

 the naked eye, because the images of the many single stars 

 overlap each other on the retina ; so that, as in the case of 

 a concentrated image, every sensible point of the latter is 

 more strongly excited." ( 106 ). 



Telescopes, unfortunately, also give, though in a much 

 less degree, an untrue or spurious diameter to stars : from the 

 examinations of William Herschel,( 107 ) however, these diame- 

 ters decrease with increased magnifying power. This acute 

 observer, with the enormous magnifying power of 6500, 

 still estimated the apparent diameter of a Lyrse at 0"'36. 

 In terrestrial objects, besides the illumination, the form of 

 the object helps to determine the smallest visual angle 

 under which it can be seen by the naked eye. Adams re- 

 marked very justly, that a long thin rod can be seen at a 

 much greater distance than could a square, whose side should 



