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 XL VI. Intelligence and Miscellaneous Articles. 



THIRD NOTE ON STELLAR SPECTRA. BY FATHER SECCHI. 



AMONG the results which we may hope to attain from the study 

 of stellar spectra, there is a very important one, which consists 

 in ascertaining whether among the stars there is one endowed with a 

 special motion of its own comparable to that of light. The difficulty of 

 this problem is still considerable, in spite of the progress of spectro- 

 metry. I was engaged upon it from the commencement of my spec- 

 tral researches in the year 1863* ; but the imperfection of the means 

 then in use did not enable me to attain any result worthy of confi- 

 dence. The stellar spectroscope having been greatly improved, I 

 have been induced by an illustrious member of the Academy, M. 

 Fizeau, to continue these researches. I have been able to attack 

 it with greater chances of success, and to solve it within certain limits 

 at any rate. But before announcing my results, the theory may be 

 briefly repeated. 



Physicists have admitted and settled that a motion of translation 

 imparted to a sounding body, which either brings it nearer or removes 

 it from the observer, can modify the pitch of the sound. M. Fizeau 

 was, I believe, the first to make this remark, and to extend it to light. 

 Formerly Doppler dwelt upon this point; and it was with the view of 

 preparing the data necessary for verifying his theory that Father 

 Sestini(in 1845) undertook at the Observatory of the Collegio Romano 

 a catalogue of the colours of the stars, and developed Doppler's 

 theory in a memoir which precedes the cataloguef. 



It follows from this theory, which has now been verified in the 

 case of sound, that when a luminous body moves away from the 

 observer with a velocity equal to the velocity of light, its colour will 

 diminish by an octave (to make use of the nomenclature of acoustics), 

 and that if the luminous point approaches the observer with a velo- 

 city equal to that of light, its colour will increase by an octave. 

 Since the interval of the waves perceptible to the eye, between the 

 red and the violet, is about an octave (the extreme waves being 

 about as 2 : 1), in the first case the star would become almost red if 

 it was violet, and would undergo an inverse change in the second 

 case. For intermediate velocities we should have proportional inter- 

 mediate variations. Thus changes in the colour of a star may indi- 

 cate that at a given moment it is approaching the observer, and that 

 at another time it is moving away from him. 



This reasoning can only lead to a practical conclusion, provided 

 that, beyond the waves visible to the eye, there are not others which, 

 in being modified, might produce visible waves. Now we know that 

 luminous bodies emit waves longer than those of the red, but shorter 

 than those of the violet — waves imperceptible to the eye, but reveal- 

 ing themselves to the thermoscope and to chemical agents. These 

 waves, by undergoing a relative elongation or shortening, might 



* Vide Bull Met. du Coll. Rom. July 31, 1863, p. 108. 

 t Mem.deir Opp. 1845. 



