NATURAL PHILOSOPHY 153 



Arago states, upon the authority of M. Babinet, that the name of 

 Barakesch, given by the Arabians to the star Sirius, signifies the star 

 of a thousand colors. He also cites various passages from the works 

 of Tycho Brahe, Kepler, and other modern inquirers, indicative of a 

 similar change of color attending the process of twinkling. M. Arago 

 asserts further, that the twinkling of the planets is a well established 

 fact. Thus observers generally, from Tycho Brahe downwards, have 

 remarked that Mercury twinkles very strongly. Venus has also been 

 observed to twinkle, and even Mars and Jupiter, though feebly. 

 Tycho Brahe has remarked that Saturn never twinkles, but this 

 opinion has been controverted by Scheiner and other observers, 

 although it is generally admitted that the phenomenon is exceedingly 

 difficult to be recognised in the case of this planet. An impression 

 has generally prevailed that the stars do not twinkle in telescopes 

 M. Arago, however asserts that this opinion is erroneous. Even a 

 few years after the invention of the telescope Simon Marius remarked 

 that, by removing the eye-piece of the telescope, the images of the 

 stars and planets appear enlarged, and exhibited rapid fluctuations oj 

 brightness and color. In 1814, Nicholson showed that a similar phe- 

 nomenon would be produced, if the eye-piece of an achromatic 

 telescope was pushed out of focus. The same inquirer also gave an 

 account of another experiment, which tended to illustrate the rapid 

 fluctuation of color which the stars undergo during the process of 

 twinkling. The telescope being adjusted to distinct vision, he applied 

 to it a smart vibration, which caused the image of the star to be trans- 

 formed into a curved line of light, returning into itself, and diversified 

 by several colors. A similar phenomenon presented itself on the 

 occasion of each successive vibration. He estimates that each color 

 occupied about a third of the whole length of the curve, and assuming 

 that he applied ten vibrations to the telescope in the course of a 

 second, he hence concluded that the light of Sirius passes through 

 thirty changes of color in a second of time. It follows, as a necessary 

 consequence of this experiment, that the stars in general shine only 

 bv a portion of their light, the effect of twinkling being to diminish 

 their brightness. It is easy to conceive, therefore, that a star which 

 is estimated to be of the seventh magnitude, because it is usually 

 invisible to the naked eye, might become distinctly perceptible if the 

 phenomenon of twinkling were to cease. 



After giving a detailed account of the circumstances which accom- 

 pany the twinkling of the celestial bodies, and of the modifications to 

 which they are subject, M. Arago next proceeds to explain his views 

 of the physical origin of the phenomenon. He maintains that the only 

 satisfactory theory which can be advanced on the subject is that which 

 connects the phenomenon with the principle of the interference of 

 light. He illustrates the latter principle by the well-known experi- 

 ment, in which two rays of light emanating in different directions 

 from a luminous point, are made to converge again by being reflected 

 from the surfaces of two mirrors, and to combine together, or to de- 

 stroy each other, according to the conditions of the experiment. If 



