278 Transactions of the Boyal Microscopical Society. 
fine longitndinal lines shown on the frustule in this photograph, 
then, Hke the coarse ones shown in the last, have all the characters 
of diffraction fringes ; hut if any douht remains, it is only necessary 
to toy with the fine adjustment, when the number of the lines and 
their distance apart will be found to vary continually, while in the 
case of the transverse striae, or of any other real lines, the number 
remains constant as long as they can be seen at all. 
Besides the principal frustule shown in these pictures, part of 
another frustule is shown in each, which affords still further illustra- 
tions of the longitudinal lines in question. I may add that, although 
I endeavoured to take the four pictures with the same power, 
trifling differences exist due to the necessary variations in the focal 
adjustment. 
As a still further illustration of the spurious longitudinal lines 
of this diatom, I add a print (marked F) of a negative magnified 
1600 diameters, made November 10, 1872, by Tolles's immersion 
yVth, in which the central frustule shows the transverse striae, 
while portions of two others exhibit longitudinal lines, similar, as 
I must suppose, to those Mr. Hickie sees. I would call attention 
on this print, and, indeed, on the two others (marked A and B) 
which show the transverse striae, to a curious series of diffraction 
lines just outside of the margin of the frustule, which appear to be 
conditioned by the transverse striae themselves. These spurious 
lines are at exactly the same distance apart as the transverse striae, 
but form a sharp angle with them. 
I send copies of all these photographs for Mr. Hickie, and also 
a set for the Koyal Microscopical Society. If the longitudinal lines 
which Mr. Hickie sees are, as I suppose, similar to those which I 
have photographed, these pictures will enable those who examine 
them to decide whether his interpretation or mine is correct. If he 
thinks he sees something of a different nature, I shall be happy to 
consider the evidence on that head when he presents it. 
These photographs will, moreover, enable those interested in 
the subject to decide another question raised by Mr. Hickie in his 
letter, viz. whether what I have photographed and described is 
really Frustulia Saxonica, and also, whether I have, as he suggests, 
"merely been wasting" my "time on a bad slide." It is quite true 
that I have been using M oiler's slides; it is also quite true that, 
like MoUer, I suppose Frustulia Saxonica to be identical with 
Navicula crassinervis. I do not pretend to any special personal 
knowledge of the proper classification of the diatoms, and derive 
my opinion entirely from my friend Professor Hamilton L. Smith, 
of Hobart College, Geneva, New York, who I suppose to be more 
thoroughly acquainted with the subject of the Diatomaceae than 
anyone on this side of the Atlantic, and who wrote me, January 9, 
1872 : " Navicula crassinervis has long been recognized as = 
