PREFACE. Vll 



The subject itself must be better understood before the 

 provisional terms now employed can be replaced by more 

 elegant or more correct expressions. 



Allowance also for certain ambiguities of expression will 

 perhaps be made, when it is remembered that this is a 

 translation of a translation, and that the author's meaning 

 may probably have suffered a little in each change of 

 language. 



The introduction of the "Anastatic" process for the 

 copying of plates, having appeared to be a highly de- 

 sirable object, if found applicable in some of the publica- 

 tions of the Ray Society, it has been tried on a small 

 scale in the present volume. The two latter plates have 

 been copied from the original lithographs by this means, 

 and it is hoped that the figures will be found sufficiently 

 good for aU their purposes ; though it cannot perhaps be 

 said that the process by which they have been copied has 

 as yet arrived at the perfection which it is much to be 

 hoped it will ultimately attain. 



G. B. 



