36 PHOTO-MICROGRAPEHS. 
have a decided advantage at this season of the 
year. 
All the photo-micrographs illustrating this vol- 
ume were made by central light, as the writer 
has given no personal attention to photography 
by oblique illumination, having been occupied 
chiefly in the practical work of making photo- 
graphic memoranda of the microscopic objects met 
with in the course of his researches relating to the 
etiology of the infectious diseases. 
For this purpose he has found central light all 
that was required, and he has had no ambition to 
follow Dr. Woodward and others in the difficult 
feats which they have accomplished in the way of 
photographing difficult test-diatoms, — e. g. Amphi- 
pleura pellucida, etc. 
Those who have this ambition will find it neces- 
sary to resort to obhque illumination, and doubt- 
less will do well to obtain Woodward’s Oblique 
Illuminator, —an apparatus for obtaining oblique 
illumination at definite angles.’ 
Artificial light may be used for making photo- 
micrographs. In localities where the sun is ob- 
scured by clouds, fog, or dense smoke, during a 
considerable portion of the time, and for persons 
who are otherwise occupied during the daytime, 
and who desire to work at night, some kind of 
artificial light will be essential. 
It is probable that the electric light will be 
found the most economical and convenient, if it 
is ever generally introduced for illuminating pur- 
1 Described in the Amer. Quart. Micr. Journal, Vol. I. pp. 268-272. 
