276 Transactions of the Boyal Microscopical Society. 
Frustulia Saxonica it is just the reverse." This statement directly 
contradicts the description of Dippel, whose measurements, quoted 
ahove, make the longitudinal lines nearly twice as coarse as the 
transverse instead of finer, and it might at first be thought that the 
two gentlemen were describing different things, but, in point of 
fact, as I stated in my "Note," these spurious longitudinal lines 
vary greatly "in their distances apart with varying obliquity of 
illumination," and also, I may now add, with varying positions 
of the fine adjustment, so that I have had no difficulty in photo- 
graphing them, as seen on the same frustule, and with the same 
objective and distance, both finer than the transverse ones as 
described by Mr. Hickie, and coarser than the transverse ones 
as described by Dippel, and in both cases they appeared to me, as 
Mr. Hickie says they did to him, " as plainly and visibly as any of 
us are ever likely to see our own faces in our looking-glasses." 
The distinctness with which these lines can be seen has nothing to 
do with the question of their objective reality, for diffraction pheno- 
mena are often quite as distinctly visible as the optical images of 
actual objects. 
The question of the real nature of these longitudinal lines 
appears to me to be one of considerable interest, because it brings 
up the matter of recognizing lines due to diffraction and inter- 
ference, when observed in the field of the microscope, and because 
these phenomena have been a fruitful source of error in the inter- 
pretation of microscopic images. I have therefore thought it worth 
while to prepare a series of photographs of Frustulia Saxonica for 
the purpose of illustrating my meaning. 
The first of these photographs (marked B) represents a frustule 
of Frustulia Saxonica adjusted to show the transverse striae. This 
frustule measured g-g-u of an inch in length, with eighty-six striae 
to the yoVo of an inch. The negative is magnified 1830 diameters 
very nearly, and to this of course the paper print closely approxi- 
mates.* 
The second photograph (marked C) represents the same frustule 
with its left end raised so as to bring it obHquely to the light. In 
its lower half, besides the transverse lines, a series of longitudinal 
lines can be seen, which give rise in places to a distinct appearance 
of dots at their intersections with the transverse striae. The nature 
of these Hnes will be best understood after a study of the third and 
fourth photographs. 
In the third photograph (marked D) the same frustule is shown 
with its left-hand angle still more elevated. The transverse striae 
have disappeared ; but in the left-hand half of the frustule we have a 
beautiful series of longitudinal lines which very closely resemble 
* Silver priiitH after mounting are usually a trifle larger than the_ negatives 
from which they are printed. 
