1 84 THE POPULAR SCIENCE MONTHLY. 



sions of the object-glass. This explains why large and faintly lumi- 

 nous surfaces, like comets' tails and the aurora borealis, can be seen no 

 better, if as well, through a telescope than by the naked eye. 



We have seen why with any object-glass a lower power than that 

 due to a two-and-a-half -inch eye-piece can not be used without loss of 

 light, and a corresponding decrease in the apparent brightness of lumi- 

 nous points seen through it. We will next consider the reasons which 

 prevent, with a given object-glass, an indefinite increase of magnifying 

 power, and, in fact, confine it to within quite moderate limits. We 

 have all seen beautiful engravings showing as well as it is possible the 

 best views ever obtained of objects like Saturn, Mars, the surface of 

 the Moon, and solar cyclones as they appear through some of the great 

 telescopes, and it must naturally occur to many to ask why a still high- 

 er magnifying power than those used can not be employed to make 

 such objects appear still larger and more distinct, for it is certainly 

 easy enough to make eye-pieces of shorter focal length than those used 

 in making the engravings just referred to, which, with a given object- 

 glass, is the only thing upon which the magnifying power depends. 



When the focal length of the eye-piece becomes reduced to one 

 sixth of an inch, the diameter of the cylinder of light-waves entering 

 the eye can only be about one thirteenth of this, or less than one sev- 

 enty-fifth of an inch, as is obvious from Diagram 6, and the eye now 

 becomes sensible of the same blurring effect that was found to occur 

 in looking through the needle-hole ; and, if a brilliant object too small 

 to have visible dimensions is observed through the telescope with such 

 an eye-piece, it will appear as a disk of considerable size surrounded 

 by one or two bright rings. 



These are the diffraction disk and rings, always seen in viewing a 

 star through a good telescope with a high magnifying power. The 

 disk is brightest at the center, diminishing somewhat in intensity to- 

 ward the edges, for which reason the diffraction disks of faint stars 

 appear slightly smaller than do those of bright stars. 



Their appearance is not simply due to the smallness of the cylinder 

 of light entering the eye through the eye-piece, but it must be remem- 

 bered that it is the diffraction disk and rings at the focus of the object- 

 glass which are viewed through the eye-piece, and not an absolute point 

 of light. The effect of this, however, can not ordinarily be distin- 

 guished in the appearance of a star, so that in practice it is found that 

 the apparent diameter of the diffraction disk of a star, expressed in 

 seconds of arc, equals about four and a half divided by the number of 

 inches in the diameter of the clear aperture of the object-glass. 



The diffraction disk becomes very important in observing close 

 double stars. It is obvious that, unless the two diffraction disks of 

 the component stars can be clearly separated, the star can not be seen 

 to be double ; to accomplish which the distance between the centers- 

 of the stars must at least equal the diameter of the diffraction disks. 



