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 

 4 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 



