10 THE AMERICAN MONTHLY [Jan 
as if the aperture of the objective were cut down to the 
same size. No fine detail would be seen with an objec- 
tive of such small aperture, and the image becomes a so- 
called “dioptric” one. Provided the cone of light from 
the condenser does not exceed this small aperture, nearly 
all the light which passes through the prism-shaped por- 
tion of the dise will be light which is diffracted by the 
object, consequently the image formed by this part is a 
so-called diffraction image. With a double-image disc 
this diffraction image is spurious, inasmuch as all fine 
periodic structure will be doubled owing to the absence 
of the dioptric beam as a component. 
A double-image disc made in this manner, but of clear 
uncolored glass throughout, ought to further test and ex- 
emplify a point in the theory of interference colors— 
viz., when the dioptric image of a diatom showed color 
due to interference, the diffraction image of the first or- 
der should show the complementary color. 
I have now such a disc, and the result, by examining 
a slide of Actinocyclus ralfsii under the microscope, has 
fully borne out the anticipation. With these diatoms, 
which show such varid and vivid coloring, it is very strik- 
ing in effect to see the two images of each diatom lying 
next to one another in precisely complementary tints. 
BIOLOGICAL NOTES. 
L. H. PAMMEL. 
BAcTERIAL DisEASES oF PLants.—During the summer 
Dr. Erwin F.Smith distributed separates of some most valu- 
able contributions to a knowledge of bacteriological diseases 
of plants constituting a reply to Dr. Alfred Fisher’s remarks 
on the existence of diseases of plants produced by bacteria. 
In addition to the large amount of original matter presented " 
here on the presence of bacteria in plant tissues and the 
pathological changes produced by them. The paper con- 
tains a large number of excellent figures. While some 
