ACKNOWLEDGMENTS 
trative material in this extremely difficult branch of photog- 
raphy has been derived from the private collections of Messrs. 
L. W. Brownell, Silas M. Lottridge, Clarence Lown, H. W. 
Shepheard-Walwyn, W. H. Fisher, and others. 
Lastly, I desire to mention the aid given by my daughter, 
Helen Ingersoll, who drew twelve of the fifteen colored 
plates and many of the smaller illustrations in the text. 
Ex ds 
New YORK, 
May I, 1906. 
viii 
