68 SPECIAL RESULTS IN THE URANOLOGICAL 



tern of waves is so far behind the other as amounts to an 

 uneven number of semi-undulations, the two systems of 

 waves strive to impart to the same molecule of ether, at the 

 same instant, equal but opposite velocities ; so that the effect 

 of their union is the repose of the molecule, or darkness. 

 In some cases it is 'rather the refrangibility of the different 

 atmospheric strata traversed by the rays of light, than the 

 different length of their paths, which performs the principal 

 part in the phenomenon. ( 13 ) 



The degree of scintillation is strikingly different in dif- 

 ferent fixed stars; not dependent solely on their altitude or 

 their apparent magnitudes, but also, it would seem, on the 

 nature of their particular light. Some, for example a Lyrse, 

 twinkle less than Arcturus and Procyon. The absence of 

 scintillation in the planets with the largest discs is to be 

 ascribed to compensation, and to the mixture of the colours 

 proceeding from different points of the disc. The disc is 

 to be regarded as an aggregation of stars, which mutually 

 restore the light neutralised by interference, and recombine 

 the coloured rays into white light. Thus traces of scintil- 

 lation are most rare in Jupiter and Saturn, but are seen in 

 Mercury and Yenus, whose diameters diminish to 4/'*4 and 

 9"'5. The diameter of Mars, at the time of conjunction, 

 may be as small as 3"*3. In the clear cold winter nights 

 of the temperate zone, the scintillation of the stars enhances 

 the impression of the lustre of the starry heavens from the 

 circumstance that, as we see stars of the 6th and 7th 

 magnitudes shine forth suddenly here and there, we are led 

 to imagine that we perceive more shining points than the 

 unassisted eye can really distinguish. Hence arises the 

 popular surprise at the few thousands of stars noted in 



