154 ANNUAL OF SCIENTIFIC DISCOVERY. 



the light from which the rays issue be homogeneous, and if the routes 

 severally traversed by them be made to differ in length by gradually 

 displacing one of the mirrors, the jooint where they meet after reflex- 

 ion will, in some positions of the movable mirror, exhibit a very vivid 

 light, while in other intermediate positions it will appear quite black. 

 The positions of the movable mirror, corresponding to which the two 

 rays thus alternately conspire together or destroy each other, will vary 

 with the color of the spectrum employed in the experiment. It re- 

 sults from this important fact, that when rays of white light emanate 

 from the luminous point, they will exhibit at their point of concourse 

 after reflexion a succession of prismatic colors, depending in each case 

 on the position of the movable mirror. It is found that similar effects 

 may be produced, if, instead of causing the routes of the two rays to 

 differ in length, the refrangibility of the media through which they 

 pass be subjected to a similar variation. It is upon these two facts 

 that M. Arago has established his theory of scintillation. In the case 

 of telescopic observations, he supposes that the rays of light which 

 enter the telescope at opposite extremities of a diameter of the object- 

 glass, may have traversed strata of the upper regions of the atmos- 

 phere, which, either from variations of density or temperature, or 

 from hygrometic causes, may possess different refractive powers. It 

 might happen from this cause that the red rays at the one extremity 

 of the diameter might totally destroy those at the opposite extremity, 

 and that the focus might pass from the normal color of white to that of 

 green, the complementary color of red. In the next instant the green 

 might be totally destroyed, and the color of the focus would, conse- 

 quently, be red ; and similar effects might manifestly be produced 

 each successive instant, by the destruction now of one color and now 

 of another color of the spectrum. Generally, the rays will only par- 

 tially destroy each other by their interference ; in which case the light 

 will still be colored at the focus, although less intensely than if the 

 destruction had been complete. M. Arago had already established by 

 experiment, that if even the twentieth part of a pencil of light were 

 extinguished by the interference of any of the homogeneous rays, the 

 light at the focus would appear sensibly colored. It would, therefore, 

 be sufficient that the strata of the atmosphere should, by reason of 

 their unequal refrangibility, affect intermittently, and in a suitable de- 

 gree, the twentieth part of the rays which the surface of a lens em- 

 braces in order that the focal point should acquire in succession the 

 different prismatic colors. " Now," says M. Arago, " if we take into 

 consideration the great length of the route traversed by the light from 

 the superior limits of the atmosphere to the object-glass of the tele- 

 scope ; if we reflect, moreover, on the small difference of refrangi- 

 bility which suffices to occasion the passage of two rays from the state 

 of accord to that of destruction, on the effect of winds, however mod- 

 erate, bringing incessantly new atmospheric strata before the tele- 

 scope, it cannot excite any surprise that in observing Sirius, a star 

 sufficiently low in our latitudes, as many as thirty changes of color in 

 a second have been noted." Having thus explained, by the principle 



