iv PREFACE 
It is a pleasure to acknowledge the kindness of various individuals 
and organizations for assistance. To the Cornell Agricultural Experi- 
ment station the author is indebted for permission to use the colored 
plates in the Chapter on Eggs, which were prepared by Professor E. D. 
Benjamin; to the various supply houses for permission to use illus- 
trations of apparatus; to Bausch & Lomb Optical Company and the 
Spencer Lens Company for permission to use parts of their manuals 
on the “Use and Care of the Microscope ”’; to the American Public 
Health Association for permission to use parts of the Standard Methods 
for the Examination of Sewage and Water, Milk, and Air, finally, to 
all those various instructors and friends with whom the author has 
come in contact and who have contributed some of the ideas reflected 
in the pages of this book. 
University oF Ixxrnois, 
Urbana, Illinois, 
August, 1918. 
