SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 107 
evidence that there were no parasites in the blood 
of the patient from whom this specimen was 
obtained; and a sufficient number of similar 
photo-micrographs of blood from different. patients, 
and drawn at different stages of the disease in 
question would prove the absence of any foreign 
elements, demonstrable with the power used, from 
the blood of yellow fever. This has been demon- 
strated by the writer in the manner indicated for 
the disease in question. For an example of the 
use of the method in those cases in which parasitic 
organisms are present, the reader is referred to 
Fig. 2, Plate I. 
POLLEN-GRAINS. 
The diversity of form exhibited by pollen- 
grains, and the curious markings, bands, and 
projections upon the external surface of the cellu- 
lar envelope of the pollen of certain plants, cause 
them to be interesting objects for the microscope. 
Not all of those, however, which it would be 
most desirable to photograph are suitable objects 
to be photographed by transmitted light, for the 
reason that the bright yellow color and compara- 
tively large size of some render them practically 
opaque. Doubtless this difficulty in the case of 
pollen-grains, the deeply colored spores of fungi, 
etc., can be overcome by special methods of 
preparation,—the use of decolorizing agents, 
mounting in media of high refractive index, ete. 
The limits of the present volume do not, how- 
ever, permit the writer to go very extensively 
