1902] MICROSCOPICAL JOURNAL. 7 
focus and bright at the lower focus. You may have a 
peculiar line on a siliceous valve to interpret. Accord- 
ing to the rule, if it be bright above and dark below, it 
is a fracture, groove, or tube; if it be dark above and 
bright below, it is a rib or other increase in thickness. I 
believe this rule to be applicable to all objects examined 
under these conditions. 
But which, after all, is the true image? The answer 
must await a definition of a true image. If an image be 
only true when it has some macroscopic analogy, then 
‘only the “white dot” image is true. If, however, an im- 
age be true when it leads to a truthful interpretation of 
structure, then the “black dot” image is also true. Both 
images, in fact, agree in contour and detail with the ob- 
ject—that is, as far as aperture diffraction effects will 
permit.—Quekett Club. 
Note on Red Rain Dust. 
G. C. KAROP. 
I received for examination from Captain OC. J. Gray, a 
small sample of Red Rain Dust, collected by him in Mel- 
bourne, Victoria, on Dec. 28th, 1896. 
The material was a cinnamon or brick-dust colored 
powder in a very fine state of division. Under the mi- 
croscope it was seen to consist almost entirely of extreme- 
ly minute amorphous particles, sparsely intermixed with 
some larger aggregations of the same, which in xylol 
- balsam and by transmitted light appeared of a dull orange 
tint. 
On going over the preparations field by field, a few 
other inorganic inclusions and still fewer organic bodies 
were to be found—the latter consisting for the most part 
of diatomaceous fragments, but in one or two cases en- 
tire valves; broken sponge spicules, some very much 
eroded, and also some curved, hayaline objects, pointed 
