PREFACE. 



IN the two series of my ' Light Science for Leisure Hours, 

 several essays from the OornJiill Magazine were included ; 

 and in one or two reviews of those works remarks 

 appeared implying that every essay I had written in serial 

 publications was included in those volumes. This is so 

 far from being the case, that more than three-fourths of 

 my essays in serials have not been published in a colleeted 

 form, and will not be. I have made it my rule to collect 

 only a few essays, carefully selected from amongst a large 

 number. The present volume, while apparently an excep- 

 tion to this rule, is not so in reality, and in fact affords 

 evidence of its operation. It is true that I here present all 

 but some seven or eight of those of my essays in the Cornliill 

 Magazine which have not hitherto been published in a 

 collected form. But, in reality, essays on Popular Science 

 intended for the Cornliill Magazine are subjected to so 

 careful an editorial process of { selection and rejection' as 

 can lead only to the * survival of the fittest.' so that such 



