312 MICROSCOPY. 
refraction of the light in passing through the external siliceous 
layer produces a distorted image upon and of the interior surface. 
In this manner distorted small hexagons may be seen along the 
lines of the larger net-work by a lens incapable of clearly dis- 
playing the minute hexagonal markings above described. 
The best results are obtained on the T. favus with a moderate 
light nearly central. 
Surirella gemma.— This beautiful form has been subjected to 
all the different conditions of illumination in my possession. 
Like other relatively thick shells the appearances presented by the 
markings vary greatly with the changing conditions of observa- 
tion. No trouble is experienced in bringing out the longitudinal 
strie, nor in making the little beauty seem to ‘ wear beads.” At 
times the beads give place to rectangles, and again after careful 
manipulation to sharply defined elongated hexagons (‘+ The Mi- 
croscope,” Carpenter, page 182). Hartnack’s hexagons as figured 
are too much elongated ; although sometimes such an appearance 
is presented when the illuminating pencil is at right angles with 
the median line, the transverse lines being less distinctly percep- 
tible. When the light is so arranged as to show every side with 
equal perfection the form of the markings is nearer that of regular 
hexagons. 
The Amici prism is found to work excellently on the Surirella, 
and when itis used with the y objective and the blue cell the 
slightly elongated hexagons are easily exhibited on an average — 
stule. os 
Aulacodiscus Kittonii.— This splendid disk is traced with o se 
angular figures evidently elevations and the spaces between om a 
lines are undoubtedly depressions. Some of the markings -i 
circular, others square, some pentagonal, some hexagonal and Le 
others heptagonal. Broken specimens of Brightwellia Johnsoni = 
with like surface markings show the line of fracture running — 
through the areola. : 
Navicula rhomboides.— Individual frustules of this species iy ; 
considerably in degree of difficulty of resolution. Some of the 
smallest valves when mounted in balsam tax the powers of excel- 
mounted dry or in balsam to yield readily transverse striæ ae 
oblique illumination direct from the lamp. Under the sa stris. 
ditions an average valve exhibits well defined longitudinal si% 
