iv PREFACE. 
filled with these photographic memoranda, and he 
constantly finds these useful for reference. Some 
also, reproduced by the heliotype process, have 
served to illustrate his papers relating to bacterial 
organisms. It is in the belief that others may find 
it worth while to avail themselves of this method 
of illustration that he has been induced to record 
the results of his own experience; and the aim 
has been, as far as possible, to simplify the tech- 
nique and popularize the art. 
In describing the plates, the Author has allowed 
himself considerable latitude, in the belief that a 
popular account of the objects described may lend 
interest to the book for some of those into whose 
hands it may fall, by reason of their unfamiliarity 
with the revelations of the microscope, and that 
such persons may perhaps have their interest 
aroused in microscopical studies, and be stimu- 
lated to further researches. 
