PBEFACE. 



THE First Series of Light Science Essays met with a 

 success so far beyond my expectations, that I should 

 have found in that circumstance alone a reason for 

 adding the present volume to the series. But I have 

 also felt a wish to publish these essays because they con- 

 tain facts collected at the cost of much labour and care- 

 fully discussed, useful, therefore, I trust, to others as 

 well as to myself, when thus gathered into a volume. 



Those who have read my former series of essays, viz., 

 ' Light Science, Series I.,' 'The Orbs around Us,' and 

 ' Essays on Astronomy,' will perceive that even when 

 I treat here of subjects already dealt with by me else- 

 where, I have been careful to avoid the repetition of 

 any statements, except those few without which a sub- 

 ject would be incomplete. For instance, it will not 

 be easy to find in my two papers on comets in ' The 

 Orbs around Us,' statements or reasoning repeated in 

 the two papers on comets in the present volume. 



However, for the most part, the papers in this series 



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