NATURAL PHILOSOPHY. 133 



period of time. This object I have actually accomplished. I have now 

 more than 20,000 observations, and the number is daily increasing. I have 

 commenced the calculations and reductions, relative to meteorological re- 

 searches, but much yet remains to be done before bringing them to a close. 

 However, as Arago has well remarked, in scientific inquiries, what had not 

 been foreseen has most frequently the lion's share. Although at first I had 

 not the remotest idea of studying the phenomenon of scintillation for its own 

 sake, still I have been led, by the force of circumstances, to consider the 

 subject; and in the course of last year I succeeded in establishing the follow- 

 ing propositions, which I have developed in a memoir published in the 

 " Bulletin de la Societe Vaudois des Sciences Naturelles:" 1. The scintilla- 

 tion of one star differs from the scintillation of another star, and in general 

 red stars scintillate less than white stars. 2. Except in the case of stars near 

 the horizon, the scintillation is very nearly proportional to the product ob- 

 tained by multiplying the astronomical refraction of the star by the thick- 

 ness of the aerial stratum traversed by the rays of light emanating from the 

 star. By adopting the theory of scintillation proposed by M. Arago, who 

 was of opinion that the phenomenon depends entirely upon the principle 

 of the interference of light, I have assigned an explanation of the first of 

 these facts, namely, that red stars scintillate less than white stars. Professor 

 Montigny, of Antwerp, who has devoted much attention to the theory of 

 scintillation, has adduced another explanation of the fact, which he conceiA'es 

 to be established by my observations. He supposes that a ray of homoge- 

 neous light like redlight, for example is less dispersed by astronomical 

 refraction, so that the pencil of light from a red star reaches the eye in a 

 less expanded state, so to speak, than a pencil of white light, and is, conse- 

 quently, liable to be partially turned aside or modified by atmospheric dis- 

 turbances. Hence, according to the theory of M. Montigny, the scintillation 

 of stars, whose light is homogeneous, ought to be more feeble than that of 

 white stars. I do not wish to pronounce an opinion here between the theory 

 of M. Montigny and that which I have proposed. However, it is not difficult 

 to see that the study of the scintillation of the stars may give rise to ques- 

 tions of great importance in regard both to optical and meteorological 

 science. Now, witli a view to this object, it would be interesting to study 

 the phenomenon in different climates and at different altitudes. Accord- 

 ingly, in the year 1856, 1 spent some time at the Hospice of Great St. Ber- 

 nard, at an altitude of 2475 metres, in order to make observations on the 

 phenomenon of scintillation, and I found it to be much less intense than on 

 the plains. Since that time Mahmoud Effendi, Director of the Observatory 

 of Cairo, has also resolved to undertake the study of this phenomenon at the 

 Observatory confined to his charge. He has announced to me that he will 

 shortly visit me at Morges, France, to confer with me on the subject, and 

 that he will then return to Egypt and commence his observations. 



The following are some of the points upon which I conceive it would be 

 important to call the attention of observers: 1. The observation on each 

 evening of the progress of scintillation, according as the stars are ascending 

 or descending with respect to the horizon. Do the stars scintillate at all 

 altitudes ? Is there any altitude at which it ceases to manifest itself ? At 

 Morges the stars in general scintillate at all altitudes, although feebly near 

 the zenith; but on the nights when the scintillation is very faint, it ceases 

 completely at a zenith distance of 45. Is it so also on the peak of Teneriffe ? 

 2 Is there a very marked difference between the scintillation of one evening 



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