SELECTION, ETC., OF OBJECTS FOR PHOTOGRAPHING. 101 
If the refractive index of a transparent object is 
exactly that of the medium in which it is sus- 
pended, and it is entirely without color, it is evi- 
dent that its presence cannot be detected either 
by the eye or upon a sensitized plate. 
The Infusoria are found in great numbers and 
variety in fresh and salt water, and especially in 
that which is stagnant and contains considerable 
organic matter. 
EUGLENA VIRIDIS. (PLATE III. Fic. 2.) 
The species shown in the figure is a common 
and widely distributed form. The variety of shapes 
assumed by the creature in its snail-like movement 
upon the surface of any object to which it attaches 
itself is well shown; but the red eye-speck and the 
green color of the endoplasm are of course lost in 
a photograph. 
SPORES OF FUNGI. 
The spores of many of the Fungi are suitable mi- 
croscopic objects to photograph, and the photo- 
graphic method could not fail to be of value to 
one especially interested in the study of the fungi. 
The deep brown color of some of these spores, 
however, causes them to arrest the actinic rays 
so completely that the photograph does not show 
plainly the internal septa which are characteristic 
features of certain species ( Conionycetes). 
The spherical or oval spores of moulds and 
mildews (Penicillium, Aspergillus, Botrytis, etc.) are 
better adapted for photography than are the more 
