NATURAL PHILOSOPHY. 149 



be produced. In connection with rarefied media it may be remarked, that 

 some of the tubes by Geissler and others have been observed to have their 

 rarefied atmospheres phosphorescent, glowing witli light for a moment or 

 two after the discharge through them was suspended. Since then, Becquerel 

 has observed that oxygen is rendered phosphorescent, i. el, that it presents 

 a persistent effect of light, when electric discharges are passed through it. 

 I have several times had occasion to observe that a flash of lightning, when 

 seen as a linear discharge, left the luminous trace of its form on the clouds, 

 enduring for a sensible time after the lightning was gone. I strictly verified 

 this fact in June 18-3.7, recording it in the Philosophical Magazine, and re- 

 ferred it to the phosphorescence of the -loud. I have no doubt that that is 

 the true explanvtion. Other phenomena, having relation to fluorescence 

 and phosphorescence, as the difference in the light of oxygen and hydrogen 

 exploded in glass globes, or in the air, were referred to, with the expression 

 of strong hopes that Becquerel's additions to that branch of science would 

 greatly explain. 



ON INTERMITTING FLUORESCENCE. 



J. Miiller has observed in platinocyanid of barium a peculiar phenomenon, 

 to which he has giving the name of intermitting fluorescence. When a strip 

 of paper is washed with a solution of the salt in such a manner that on evap- 

 oration the surface appears covered with a layer of delicate green crystals, 

 and then exposed in a dark room to the spectrum produced by a flint glass 

 prism, aided by a lens of long focal distance, almost the whole portion on 

 which the blue rays fall appears blue. In this blue portion, however, three 

 isolated green fluorescent bands appear. The middle of one of these bands 

 corresponds to Frauenhofer's line G ; the two others lie between G and F. 

 The centres of these bands correspond" to the wave-lengths 0'0004G'2 nim , 

 0-0004 10 mm , 0'00430 mra . From this it appears that rays of these wave- 

 lengths produce fluorescence, while those of intermediate wave-lengths pro- 

 duce none. An uninterrupted green fluorescence begins at that portion of 

 the spectrum which corresponds to a wave-length of about 0'000410 mm . No 

 similar phenomenon has hitherto been observed. Fogg. Ann., civ., 649. 



NOTE ON THE POLARIZATION OF THE LIGHT OF COMETS. 



The following note, communicated by Sir David Crcwster to the French 

 Academy, is published in the L. H. and T>. Philosophical Macjazine, for April 

 1859, p. 311. Although there can be no doubt as to the accuracy of the 

 observations of M. Arago, on the indications of polarization discovered by 

 him in the light of the comets from 1819 to 1S35, there is nevertheless noth- 

 ing impossible in the supposition that the light may have been polarized after 

 arriving in the terrestrial atmosphere. In fact, when we consider that light 

 is polarized by refraction in passing through the coats of the eye, that it is 

 polarized by refraction at the four or six surfaces of the object-glasses of an 

 astronomical telescope, and also in passing through the surfaces of its eye- 

 piece, and lastly, that the light of celestial bodies undergoes a slight polar- 

 ization by the refraction of the atmosphere, we are compelled to admit that 

 the problem of the existence of polarized light in the light of comets is not 

 solved. 



I arn not aware that those who have observed traces of polarization in the 

 light of comets, have noticed the direction of the plane in which it has been 



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