THE SCINTILLATION OF THE STARS. 225 



as probably affording the best means of comparing the values obtained 

 at different places on the globe. 



While scintillation is readily appreciable for stars of the first mag- 

 nitude, it is much less so for those of a lower grade; and thus in 

 considering the stars as they become less and less brilliant, we arrive 

 finally at those in regard to which scintillation is wholly unapprecia- 

 ble; though this limit itself varies from one day to another, according 

 as the general scintillation is stronger or weaker. Here, then, we 

 have still another criterion for a comparison of the scintillation. It 

 •suffices to say to what degree of magnitude the stars appear to scin- 

 tillate; and as we have seen that the height of stars above the horizon 

 has a great influence on the intensity of the phenomenon, it will be 

 always necessary to cite by name some of these stars, and to indicate, 

 besides, what is their height above the horizon, or at least the hour 

 of the observation. 



5. Variable stars. — Of all the stars which I have observed, alpha 

 of Orion is that whose scintillation has appeared to me most irregular, 

 but we know that the brightness of this star is not always the same. 

 Now, in regard to the variable stars, we know at most but the dura- 

 tion of their period, and hence their scintillation is also an interesting 

 phenomenon for our study. It would be interesting, especially in 

 relation to tj of the Ship, the singular variations in the brightness of 

 which have so much surprised astronomers for 30 or 40 years past. 

 Unluckily this star only begins to be visible at 31° of north latitude, 

 and it is necessary to proceed nearly to the equator before it can 

 attain an elevation of 30° above the horizon; that is, a height sufli- 

 €iently great for the satisfactory study of its scintillation. This in- 

 vestigation must be resigned, therefore, to those who have an oppor- 

 tunity of observing the skies of the other hemisphere. 



6. Scintillation of planets. — It is the general belief that the planets 

 have no scintillation, or next to none. Yet Venus and Mars have 

 often a very perceptible scintillation. In some rare instances I have 

 even detected a slight scintillation in Jupiter and Saturn. It would 

 be of some importance to those who seek to explain the phenomenon 

 of scintillation to know whether, in fact, the scintillation of these 

 planets is ever very distinct, and the inquiry may be properly recom- 

 mended to those who may be placed in such physical circumstances 

 as render the general scintillation unusually strong. Perhaps Jupiter 

 and Saturn might then be seen to scintillate with great distinctness. 



7. Accidental observations. — In order, finally, to complete the study 

 of this phenomenon, it will be necessary not to neglect exceptional 

 circumstances, among others the observation of scintillation in case 

 of an aurora borealis, whether in regard to the stars Avhich seem im- 

 mersed in the light of the aurora or those situated in other regions 

 of the firmament. During six years I have not been able to make, 

 at Morges, any observation of this kind. As to observations of the 

 scintillation by those stationed on mountain heights, M. Piazzi Smyth 

 recollects that scintillation appeared very weak as observed from the 

 summit of the Peak of Teneriffe, and I constantly realized the same 



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