OF THE STABS. 99 



how the momentary coloured effulgence of a star may be fol- 

 lowed by its equally instantaneous disappearance or sudden 

 obscuration, in an atmosphere composed of ever-changing 

 strata of different temperatures, moisture, and density. The 

 undulatory theory teaches us generally that two rays of light 

 (two systems of waves) emanating from one source (one centre 



tions du bord du soleil sont de 4" a 5", et peut-etre dea 

 pieces qui manquent, done encore effet de 1'interference des 

 rayons." 



On the causes of the scintillation of the stars. 



" The most remarkable feature in the phenomenon of the 

 stirs' scintillation is their change of colour. This change is 

 of much more frequent occurrence than would appear from 

 ordinary observation. Indeed, on shaking the telescope the 

 image is transformed into a line or circle, and all the points of 

 this line or circle appear of different colours. We have here 

 the results of the superposition of all the images seen when 

 the telescope is at rest. The rays united in the focus of a 

 lens, vibrate in 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 destroy 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 remains 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 remembered that tlue disc may be regarded as an aggrega- 

 tion of stars, or of small points, scintillating independently of 

 each other, while the images of different colours 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 disc surrounded by a series of luminous rings* 



H2 



