226 IN STARRY REALMS. 



trace of the feeblest twilight glow. An ordinary cloud 

 would, of course, be invisible except as concealing the 

 stars ; no beams of light fall upon it ; there is nothing to 

 render it luminous. So, too, the meteoric streak will 

 often pass instantly into invisibility, but, as I have said, 

 this is not always the case. There is a well-authenti- 

 cated instance in which the trail of a superb meteor 

 remained visible for nearly an hour. I have en- 

 deavoured up to the present to explain the various 

 phenomena presented to us in the fall of a meteor, but 

 here, for the first time, we have to note a circumstance 

 for which it is not easy to account. We can explain why 

 it is that the long meteoric cloud should be there, but we 

 cannot so easily explain why we should be able to see it. 

 Whence comes this beautiful pearly luminosity ? Ii 

 seems that the meteoric dust must glow with some in- 

 trinsic luminosity. I have only heard one attempt to 

 offer a rational explanation of the true character of this 

 interesting phenomenon. It is as follows. 



There are certain substances which are called phos- 

 phorescent, because they possess the power of actually 

 absorbing and retaining the light to which they have 

 been exposed, and then gradually dispensing it after- 

 wards. Phosphorescent materials have some useful appli- 

 cations as ingredients in the so-called " luminous paint.'* 

 A gate covered with this preparation will absorb the sun- 

 light during the day, and during the night will radiate 

 forth its store in a feeble glow. Clock faces have been 

 similarly illuminated, and match-boxes are made so that 

 the hand shall be guided to them in the dark by a radiance 

 more ghostlike than beautiful. The persistent streak of 



