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pher, two rays of hymogeneous light, arriving at the same 

 point of space by two ways little different, add or neutralize 

 themselves, according as the difference of the two ways has 

 this, or that value. The differences, which accord with the 

 neutralization of rays of different tints are so sensibly un- 

 equal, that the result of the interference, or mixture of two 

 white pencils of light, should always be accompanied by a 

 manifest coloration. Experience has moreover shown us 

 (see Annates de Chimie etde Physique, torn, i, p. 199), that, in 

 seeking the place where two pencils are capable of influ- 

 encing each other, it is not sufficient to consider merely the 

 difference of the ways they have traversed ; but it is neces- 

 sary also to attend to the unequal refractive powers of the 

 mediums they have traversed. This admitted, it is easy 

 to demonstrate, that the rays emitted from the same place, 

 collected in the focus of a small lens, vibrate in concert, or 

 unite together, if they have all traversed mediums of the same 

 density, or equal refractive power ; and the same mode of 

 reasoning shows, that, on the contrary, an unequal refractive 

 power must, according as we suppose it more or less consi- 

 derable, occasion, in the same focus, the neutralization of 

 some class or other of coloured rays. If we apply these 

 considerations to the twinkling of stars, we shall find that, if 

 all the rays of light, which arrive at the different parts of the 

 pupil, constantly traverse atmospheric strata of equal den- 

 sity, the image of the star will always be equally intense, 

 and of the same colour ; while in the contrary supposition, 

 it's colour and brilliancy may vary every instant. The 

 chances of twinkling will be much less numerous for a star at 

 the zenith, under the same circumstances, than for one but 

 little above the horizon. In our climates they will be less 

 than under the tropics, where heat is more uniformly distri- 

 buted in the strata of the atmosphere. The variations of 

 intensity will be more easily perceived in stars of the first 

 magnitude, where they will be attended with a change of 

 colour more striking than in small stars $ and in white stars, 



