277 Transactions of the Royal Microscopical Society. 
those shown in Dippel’s woodcut above referred to. Such longi- 
tudinal lines as these. I suppose, Mr. Hickie himself has observed and 
recognized as spurious, for he says in his letter — “ I had also no 
difficulty in bringing into view those wide-spaced, spurious lines 
alluded to by Dr. Woodward.” 
Let us observe the character of these lines. They run parallel 
to the midrib, handsomely following its sweeping curves, and it 
will be seen that they become progressively closer and closer 
together from the midrib towards the margin of the frustule. It 
will also be seen that they do not terminate at the edge3 of the 
frustule, but sweep off into the open space outside, where they 
form a series of rhombic figures by crossing a fresh series of diffrac- 
tion lines conditioned by the margin of the frustule, as the former 
series was by the midrib. These characters would be sufficient to 
show these lines to he spurious, but if, now, the fine adjustment be 
toyed with, or the illumination changed, or both, they pass, by the 
most insensible transitions, into new combinations, one of which is 
shown in the next photograph. I would also call the attention of 
those who examine this photograph to three shadows on the left of 
the lower part of the frustule, which are cast by three out-of-focus 
fragments of dirt adhering to the under surface of the glass cover 
of the preparation. Each of these shadows is surrounded by a 
series of lines due to diffraction and interference. The lower two 
sets of these lines especially very closely resemble the longitudinal 
lines on the frustule in number, distance apart, distinctness, and 
general character. 
In the fourth photograph (marked E) the same frustule is 
shown standing vertically with the light coming from the right 
side of the picture. On both sides of the frustule, but especially 
on the right side — the left being in shadow — there are a series of 
fine longitudinal lines, which I suppose to be similar to those Mr. 
Hickie has described. They are rather closer together than the 
transverse striae (counting eleven, on the negative, in the space 
occupied on the negative of the photograph marked B, by ten of the 
transverse striae), and their distance apart is more equable than is 
the case with the longitudinal lines in the last photograph. Still, 
if the paper print be examined with a hand-lens, I think it will be 
plainly seen, as can readily be measured on the negative, that the 
lines become progressively closer towards the margin, those nearest 
the midrib being farthest apart. It will also be seen, on both sides 
of the frustule, and especially at its upper end, that these longi- 
tudinal lines are not limited to the surface of the frustule, but pass 
off into space outside, where they cross a fresh series of beautiful 
diffraction lines conditioned by the margin of the frustule. I would 
particularly invite attention to this last series of lines, especially as 
seen towards the top of the right-hand margin of the frustule. The 
