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. AV'e 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.