ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
367 
Tests to determine the reality of the ghosts : — 
(1) They usually occur whtn a small cone of illumination is em- 
ployed. 
(2) They must be an integral multiple of some real structure : thus, 
if the real structure is 12,000, the ghost may be 24,000, 36,000, 48,000, 
&c., but it can never be 18,000 or 30,000. 
(3) The ghosts invariably have a focus differing from that of the 
true structure. 
The “ black and white dot ” is a term used to express the fact that 
when an object — e.g. a siliceous plate — is viewed under the Microscope, its 
edge assumes either a black or white appearance, according to changes in 
focus ; but when the edge is an inner edge of a hole, and the hole is 
very minute, the black edge on one side of the hole will meet the black 
edge on the opposite side, and the hole will appear as a “ black dot ” ; 
but when the focus is arranged so as to give a white edge, then the hole 
becomes a white dot. Although primarily applied to diatoms, the term 
is applicable to all minute microscopical objects, such as bacteria, hairs, 
flagella, and the edges of objects generally. This phenomenon is found 
to depend upon the aperture of the objective ; for the greater the aperture 
the easier it is to obtain a black dot. When, however, the hole becomes 
excessively minute, a black dot is no longer attainable, and we have to 
content ourselves with the white dot appearance. There is nothing in 
the theory of microscopic vision, as at present enunciated, to explain why 
a larger aperture is required to resolve the black than the white dot. 
In dealing with the limit of microscopic vision, the question arises 
whether we mean the black or white dot limit, for there must be two 
limits. Again, which is the more correct picture ? Moreover, as these 
images occur at different foci, which is the correct focus ? and as it 
depends on the adjustment of the objective, which is the correct adjust- 
ment ? 
Another problem awaiting solution relates to dark-ground illumina- 
tion. This illumination is best obtained by placing an opaque stop at 
the back of the condenser to stop out an axial cone of greater aperture 
than that of the objective. It is found in practice that, when the 
ground is strictly dark, the resolving limit of all objectives is lowered. 
When the stop at the back of the objective is hardly large enough, the 
ground assumes a pearly appearance ; in this case the resolving limit 
is at its maximum. 
(6) Miscellaneous. ; 
Vessel for Treatment of Paraffin Sections with Staining and 
other Solutions.* — Dr. L. Buscalioni describes a vessel (figs. 53-57) 
which is a modification of that devised by D. Caro for the treatment 
of paraffin sections. The receiver is a rectangular glass jar measuring 
8 cm. high, 6 • 5 cm. broadband 8 • 3 cm. long. The lid is made of ebonite 
and is 8*8 cm. long by 7 cm. broad. In it are twelve openings, each 
about 4 cm. long, and sufficiently wide to take two slides placed back to 
back. On the under surface of the lid is a groove for the purpose of 
fitting the lid on to the receiver. The lid is covered with a cap made of 
* Malpighia, xi. (1897) pp. 458— 60| (5 figs.). Zeitsch. f. wiss. Mikr., xiv. (1898) 
pp. 442-4. 
1898 2 0 
