xiv PREFACE 



point when it came under the survey of 

 Helmholtz, and then I have given an account 

 of the contributions made by himself. Few 

 are able to tread in the footsteps of Helm- 

 holtz, even although these are carefully pointed 

 out by a competent guide; but this volume 

 may at all events give an outline of the paths 

 along which he trod. 



The proofs have been read by Mr J. L. 

 Galbraith, of the Library of the University of 

 Glasgow, to whom I owe my thanks ; and I 

 have gratefully to acknowledge much valu- 

 able assistance and friendly criticism from Dr 

 Cargill G. Knott, Lecturer on Mathematical 

 Physics in the University of Edinburgh. 



The characteristic portrait was most kindly 

 lent by Lord Kelvin. It was taken by Mr 

 Henderson, a student of physics at that time, 

 and it represents Helmholtz at his lecture table 

 on July 7, 1894, a few days before his final 

 illness: 



JOHN G. M'KENDRICK. 



UNIVERSITY OF GLASGOW, 

 September, 1899. 



