392 Dr. Gladstone on the Periods and Colours 



Intensity. — Helmholtz has shown that light of almost any 

 degree of refrangibility, if very intense, gives a sensation of 

 whiteness; and perhaps of all rays, the blue exhibits this in the 

 most striking manner. 



This will account most satisfactorily for two separate facts 

 already adverted to. The one is the apparent paradox of a white 

 meteor shedding a blue radiance. The intensely brilliant cloud 

 over Hampstead was doubtless emitting blue rays in the great- 

 est proportion, but on account of its brightness the blue tinge 

 could not be observed except as reflected from distant objects. 

 The other is the phsenomenon so frequently observed by M. 

 Coulvier Gravier, of a white meteor becoming bluish or reddish 

 as it approached the horizon. Indeed it might have been pre- 

 dicted that many a slightly-coloured meteor would appear white 

 when in the zenith, and thus nearest to the spectator ; and that 

 as it passed to a distance, its luminosity being diminished, its 

 proper colour would become evident. This may also be one 

 reason why so many meteors are called blue by English obser- 

 vers, while under the clearer skies of the Continent and China 

 they are designated white- blue. 



All who have been in the habit of observing the solar spec- 

 trum, must have remarked how the apparent colour of the space 

 between the fixed lines D and E varies with the intensity of the 

 light ; what appears green when diffused light is examined, is 

 yellow when the rays come direct from the sun itself. Proofs 

 might easily be multiplied of the tendency of yellow light, when 

 rendered dull, to give the impression of green. 



This suggests a simple physical cause for what is frequently 

 noticed in the Paris observations, — a yellowish meteor becoming 

 more or less green as it passes away from the spectator. 



Contrast. — The apparent colour of an object is always affected 

 to a greater or less degree by the colours of surrounding objects. 

 This source of error must also come into play with meteors ; and 

 perhaps one of the results will be, that the sparks or fragments 

 thrown off from the luminous globe may appear more distinct in 

 colour than they really are. 



Period and Colour. — If the cosmical theory of the origin 

 of meteors be the correct one, there would be no improbability 

 in supposing that the stream of little bodies revolving round the 

 sun, and crossing our orbit at one period of the year, should 

 differ in composition from those that cut our path at another 

 period. If different in composition, they would probably exhibit 

 different colours in combustion. It became an interesting point, 

 therefore, to examine whether the catalogues show any indication 

 of this. 



On examining the Chinese records, we find that the predomi- 



