iv PREFACE 



Although the general aim, and therefore the principal title, 

 of this book remain the same as in the first edition, it has been 

 entirely rewritten and much extended. The chief differences, 

 aside from improvement in details, consist in two things: First, 

 there is much greater insistence upon mechanical neatness in 

 experimentation, upon effective exposition, and upon scientific 

 logic. Second, the emphasis is thrown, for advanced or college 

 work, not upon qualitative results obtained by students from 

 apparatus of their own making, but upon quantitative results 

 obtained from practically accurate, or normal, apparatus manu- 

 factured expressly for its particular work. The reasons for this 

 change will be made plain a few pages later. I wish here simply 

 to emphasize its presence and its importance. 



The first edition of the book had the advantage of criticism 

 in manuscript by Professor C. R. BARNES, and much that he 

 suggested is retained in this edition. I have had, as before, the 

 constant co-operation of Mr. E. J. CANNING, the skilled Head 

 Gardener of Smith College, in the effort to find the best mate- 

 rials for educational work. I have received, also, much aid from 

 many of my loyal students, some of whom are mentioned by 

 name in the following pages; but I am especially indebted to 

 Miss SOPHIA ECKERSON, whose aid and criticism have been 

 constant and valuable. To the BAUSCH and LOME Optical Com- 

 pany I owe the use of the cuts illustrating my normal apparatus 

 made by them, and they have permitted me also to copy freely 

 from their copyrighted descriptive catalogue of this apparatus. 

 To those who have thus co-operated in making this work more 

 serviceable, I wish to express the grateful thanks not only of 

 myself, but of all who may find the book of use. 



WILLIAM F. GANONG. 



SMITH COLLEGE, June, 1908. 



