AUTHOR'S PREFACE TO THE SECOND EDITION ix 



described in considerable detail, has been critically examined 

 with reference to concrete examples. It must, however, be ex- 

 pressly remarked that for the most part only methods of general 

 significance could receive consideration, while methods worked 

 out for special purposes have at most been briefly referred to, 

 especially when satisfactorily dealt with in easily accessible 

 original treatises. 



Many new drawings, an index, and the appendix indicating 

 where materials and apparatus may be obtained, will add to the 

 value of the book. 



I have taken great care in the selection of the research material 

 recommended for the experiments. It will be found that not only 

 material available in summer has received attention, but also 

 material which may suitably be employed in winter. It is 

 precisely in the less favourable season of the year that the 

 proper choice of material is most important. 



The number of experiments which I have made during the 

 last four years to test methods of research has been very large 

 I have also devised many new pieces of apparatus, which are 

 described and figured in this edition. 



It must further be remarked that the literature appearing 

 after the middle of 1894 could not receive attention. 



A French translation of the First Edition of my book, by Dr. 

 Micheels of Ypres, was published at Paris in 1890. 



My best thanks are due to the publisher for the great care he 

 has taken in the get-up of the book. 



W. DETMER. 



JENA, April, 1895. 



