SCINTILLATION OF THE STARS. 75 



these alterations are more intense in reality than they appeal 

 to the naked eye ; for when the several points of the retina 



harmony or at variance with one another, and increase or destroy one 

 another according to the various degrees of refraction of the strata 

 through which they have passed. The whole of the red rays alone can 

 desti'oy one another, if the rays to the right and left, above and below 

 them, have passed through unequally refracting media. We have used 

 the term alone, because the difference of refraction necessary to destroy 

 the red ray is not the same as that which is able to destroy the green 

 ray, and vice versa. Now, if the red rays be destroyed, that which re- 

 mains will be white minus red, that is to say, green. If the green, on 

 the other hand, be destroyed by interference, the image will be white 

 minus green, that is to say, red. To understand why planets having large 

 diameters should be subject to little or no scintillation, it must be remem- 

 bered that the disk may be regarded as an aggregation of stars or of 

 small points, scintillating independently of each other, while the images 

 of different colors presented by each of these points taken alone would 

 impinge upon one another and form white. If we place a diaphragm 

 or a cork pierced with a hole on the object-glass of a telescope, the 

 stars present a disk surrounded by a series of luminous rings. On push- 

 ing in the eye-piece, the disk of the star increases in diameter, and a 

 dark point appears in its center; when the eye-piece is made to recede 

 BtiU further into the instrument, a luminous point will take the place of 

 the dark point. On causing the eye-piece to recede still further, a 

 black center wiU be obsei'ved. If, while the center of the image is 

 black, we point the instrument to a star which does not scintillate, it 

 will remain black as before. If, on the other hand, we point it to a scin- 

 tillating star, we shall see the center of the image alternately luminous 

 and dark. In the position in which the center of the image is occu 

 pied by a luminous point, we shah see this point alternately vanish and 

 reappear. This disappearance and reappearance of the central point 

 is a direct proof of the variable interference of the rays. In order to 

 comprehend the absence of light from the center of these dilated im- 

 ages, we must remember that rays regularly refracted by the object- 

 glass do not reunite, and can not, consequently, interfere except in the 

 focus; thus the images produced by these rays will always be uniform 

 and without a central point. If, in a certain position of the eye-piece, 

 a point is observed in the center of the image, it is owing to the inter- 

 ference of the regularly refracted rays with the rays diffracted on the 

 margins of the circular diaphragm. The phenomenon is not constant, 

 for the rays which interfere at one moment no longer do so in the next, 

 after they have passed through atmospheric strata possessing a varying 

 poAver of refraction. We here meet with a manifest proof of the im- 

 portant part played in the phenomenon of scintillation by the unequal 

 refrangibility of the atmospheric strata traversed by ra} s united in a 

 Tery narrow pencil." 



"It follows from these considerations that scintillation mast necessa- 

 rily be referi'ed to the phenomena of luminous interferences alone The 

 rays emanating from the stars, after traversing an atmosphere composed 

 of strata having different degi-ees of heat, density, and humidity, com- 

 bine in the focus of a lens, where they form images perpetually chang 

 ing in intensity and color, that is to say, the images presented by sciii 

 tillation. There is another form of scintillation, independent of the fo 

 cus (if ih i telescope. Tlie explanations of this phenomenon advaaceJ' 



