MICROSCOPY. 311 
more regular near the margin of the diatom or appears so by 
reason of the simpler character of the structure in that part. 
They are easily seen with the J; on any part of every specimen 
studied, but are unusually distinct between the ‘four spherules” 
on the inner plate looking through the largest openings in the 
. outer plate; or may be rendered still more distinct on shells with 
the concave side up. They are more difficult to be seen on the 
outside crust with the high powers used because of its greater 
opacity. Deductions from focal changes with reference to the 
various markings lying in different focal planes corroborate the 
conclusions above expressed. 
The disks examined are on Mdller’s Probe-Platte and on a slide 
prepared by Mr. Wells. 
Hyalodiscus subtilis Bailey —On this beautiful little shell the 
‘engine rulings” are readily seen with almost any illumination, 
and the inevitable concomitant of intersecting lines, whether real 
or illusory beading are displayed. When we use monochromatic 
light the whole scene is changed. The hyaline portion of the disk 
is instantly resolved into perfectly well defined hexagons, radiat- 
ing from the central nucleus. The central part because of its 
greater depth and complexity is only resolved into irregularly 
Shaped spaces of a more or less hexagonal form. Every one of 
the five beads usually seen represents the centre of an hexagonal 
plane exactly as in Pleurosigma angulatum. 
The hexagons are well defined with a power of 7,000 diameters. 
They may also be seen with lamp or daylight. 
Triceratium favus.—The two sets of markings on this fasci- 
nating object certainly lie in different focal planes (see Carpenter, 
“The Microscope,” 4th ed., p. 282 and note) and probably ‘‘ be- 
long to two distinct layers.” The coarse hexagonal ridges are 
found to project from the outer or convex surface, and the inner 
plate bears the minute markings. This is proved by the fact that 
the fine markings show decidedly the plainest on valves that are 
mounted with the interior surface uppermost. 
Under this superior objective the finer markings like. the larger 
are distinctly faveolate. Their hexagonal structure is easily seen 
even with lamp illumination. Wh tively thick 
shells, possessing 4 complex structure, like the ee in question, the 
necessity for avoiding errors caused by too intense or by exces- 
Sively oblique light becomes at once apparent. The unequal 
