222 THE POPULAR SCIENCE MONTHLY. 



fragments, so to speak, of one or possibly two similar rings a lit- 

 tle farther from the disk, and visible, perhaps, only by glimpses. 

 These "diffraction rings" arise from the undulatory nature of 

 light, and their distance apart as well as the diameter of the cen- 

 tral disk depend upon the length of the waves of light. If the 

 telescope is a really good one, and both object glass and eyepiece 

 are properly adjusted, the disk will be perfectly round, slightly 

 softer at the edge, but otherwise equally bright throughout ; and 

 the ring or rings surrounding it will be exactly concentric, and 

 not brighter on one side than on another. Even if our telescope 

 were only two inches or two inches and a half in aperture we 

 should at once notice a little bluish star, the mere ghost of a star 

 in a small telescope, hovering near the pole star. It is the cele- 

 brated "companion," but we shall see it again when we have 

 more time to study it. Now let us put the star out of focus by 

 turning the focusing screw. Suppose we turn it in such a way 

 that the eyepiece moves slightly outside the focus, or 

 away from the object glass. Very beautiful phenom- 

 ena immediately begin to make their appearance. A 

 slight motion outward causes the little disk to expand 

 P erce ptibly, and just as this expansion commences, a 

 bright-red point appears at the precise center of the 

 disk. But, the outward motion continuing, this red center disap- 

 pears, and is replaced by a blue center, which gradually expands 

 into a sort of flare over the middle of the disk. The disk itself 

 has in the mean time enlarged into a series of concentric bright 

 rings, graduated in luminosity with beautiful precision from cen- 

 ter toward circumference. The outermost ring is considerably 

 brighter, however, than it would be if the same gradation applied 

 to it as applies to the inner rings, and it is surrounded, moreover, 

 on its outer edge by a slight flare which tends to increase its ap- 

 parent width. Next let us return to the focus and then move the 

 eyepiece gradually inside the focal point or plane. Once more 

 the star disk expands into a series of circles, and, if we except 

 the color phenomena noticed outside the focus, these circles are 

 precisely like those seen before in arrangement, in size, and in 

 brightness. If they were not the same, we should pronounce the 

 telescope to be imperfect. There is one other difference, however, 

 besides the absence of the blue central flare, and that is a faint 

 reddish edging around the outer ring when the expansion inside 

 the focus is not carried very far. Upon continuing to move the 

 eyepiece inside or outside the focus we observe that the system of 

 rings becomes larger, while the rings themselves rapidly increase 

 in number, becoming at the same time individually thinner and 

 fainter. 



By studying the appearance of the star disk when in focus and 



