1902} MICROSCOPICAL JOURNAL. 9 
are, firstly as above—viz., extremely finely comminuted 
particles of red earth with various inclusions, raised by 
some tornado and carried, it may be, for immense dis- 
tances from its place of birth; and, secondly to the some- 
times extraordinary development in a very short space 
of time of the lowly protophyte known as Spherella (Pro- 
tococcus) pluvialis, which, besides its usual green, in cer- 
tain peculiar and not very well ascertained conditions 
sometimes assumes a red color, and was in this state form- 
erly differentiated as Hematococcus. Hither this and al- 
lied forms such as, for instance, the hardly separate one 
known as S. nivalis, causing ‘““Red-snow,” or Bacillus pro- 
digiosus, or other chromogenous bacteria, and some of 
the Palmellacezx, as P. cruenta, occasionally appear with 
extreme suddenness, and among the unlearned give rise 
to no little alarm, and to dire prognostications of war, 
famine, and sudden death.—Quekett Club. 
Double Image Disks and Complementary 
Interference Colors. 
J. RHEINBERG. 
Some years ago I was making experiments with mul- 
tiple-color illumination, using discs above the objective , 
and, in making one or two such discs out of colored glass, 
Messrs. Zeiss, of Jena, observed a displacement of the 
image of the central portion of the color disc relatively 
to that of the peripheral portion. They then made a 
‘double-image color disc on purpose. 
A prism-shaped disc of green glass had a central hole 
perforated in it, in which was inserted a smaller disc of 
red glass with plane parallel snrfaces. Consequently the 
light passing through the prism-shaped part is shifted 
slightly as compared with that passing through the cen- 
tre, and we get a separate image formed by each part. 
That formed by the central portion is an image the same 
