CORRESPONDENCE. 
35 
nervis and Nav. cuspidata is something very different from saying 
that the two things are identical, as Moller says in his printed expla- 
nation accompanying his Diatomeen-Probe-Platte : — 
" 18. Navicula crassinervis Breb. = Frustulia Saxonica Rabh." 
Let the reader compare Dr. Eabenhorst's drawing of Frustulia 
Saxonica* with the drawing of Nav. crassinervis in Smith's Synopsis. 
I may here remark that Dr. Schumann's own drawing of Nav. cras- 
sinervis (plate iv. fig. 57) is markedly different from Fr. Saxonica. 
It is also very strange that Dr. Rabenhorst himself, when he saw 
them both in quick succession at my rooms in Dresden, had no sus- 
picion that they were identical. Surely he ought to know. And now 
to revert to what I have said about position. 
So far as my experience goes Navicula crassinervis is not readily 
resolved unless it lies perfectly horizontal. 
Nav. rhomhoides is perfectly resolved only when the (apparently) 
right-hand apex has an elevation of about 9°. 
With Frustulia Saxonica it is exactly the reverse. It is fully 
resolved only when the (apparently) left-hand apex is elevated about 
15°. And this will serve to distinguish them. In JRhomhoides also the 
transverse lines are much closer than the longitudinal, whereas in 
Frustulia Saxonica it is just the reverse. Their general contour, 
again, is different. In JRhomhoides the marginal edge runs pretty 
straight from the apex to the centre, where the frustule exhibits an 
obtuse angle. In Frustulia Saxonica the marginal edge forms a fairly 
continuous curve, and the ends are considerably sharper. There are 
also some characteristic differences in their central knot, which, how- 
ever, are less apparent, 
I have entered into these minute particulars lest anyone should 
suspect that Messrs. Seibert, Rabenhorst, Lindig, and myself have 
been confounding Frustulia Saxonica wdth Bhomhoides. 
In conclusion, I will beg Dr. Woodward not to interpret any word 
or sentence I have here used as intended to detract from his well- 
earned reputation as one of our greatest living microscopists, or as 
provocative of controversy, which I cordially detest. If he finds any 
expression that can bear such an interpretation I shall wish it unsaid: 
For my own part, I see no reason why matters microscopical may not 
be talked over as amicably as the state of the weather or the prospects 
of a good harvest. 
Yours faithfully, 
W. J. HlOKIB. 
Zeiss's and English small-angled Objectives. 
To the Editor of the ^Monthly Microscopical JournaV 
May 18, 1875. 
SiE, — At the last meeting of the Royal Microscopical Society, one 
of the Secretaries, Mr. Slack, read a very interesting paper on the 
above subject, and I have no doubt that all microscopists will be 
thankful for such a subject having been brought forward afresh. 
* ' Die Siisswasser-Diatomaceen,' plate vii. fig. 1. 
