On “ Navicula crassinervis,” &e. By W. H. Dallinger. 7 
sharply and clearly separate, and the continuity of this is unbroken 
from end to end. 
If this form be studied with a good } (dry) of an angle of 
aperture of about 95°, and if the stage of the microscope be 
moderately thin, and all sub-stage gear be taken away, and the light 
from a good lamp condensed by a “ bull’s-eye” of about 5-inch 
focus be sent in at an angle of about 30° with the under surface 
of the slide, and the image of the flame be focussed on the centre 
of the object, this specimen of Rhomboides can be exquisitely 
resolved, on a black ground, with a third, or even a fourth eye- 
piece; the frustule itself, especially if seen amongst brilliantly 
white diatoms, assumes a pale sapphire tint, and the hemispheres 
with skilful management are beautifully developed. 
Taking these facts together, then, and guided by the decision 
announced by Professor H. L. Smith and Mr. Kitton, we may 
safely conclude that N. rhomboides is a diatom very permanent in 
its general form and characteristics, but eatremely variable in its 
size, and the tenwty of the ultimate structure of its silictous 
Srustules. 
But from this, a corollary inevitably follows. N. rhomboides 
ig a most uncertain and unreliable test. Unless the same mount- 
ing, and in many instances the same frustule, be used, in testing 
the capacity of a given lens, the most incongruous issues may 
result. To be told that a certain glass will resolve “ N. rhomb- 
oides,” may mean that it will resolve E, Plate CLXVL., or that it 
will resolve A, Plate CLX V.—quite a different result, and no guide 
as to whether A’s or B’s lens 1s better or worse than mine of the 
same power, unless the same slide of frustules be employed. 
To those accustomed to the use and comparison of diatomaceous 
test-objects for practical purposes, this is no new inference, as 
Mr. Kitton has recently so profitably told us.* But it is so palpable 
an instance that it may carry conviction. 
DESCRIPTION OF PLATES CLXY. AND CLXVI. 
Fig. A.—WNavicula crassinervis x 800 diameters, showing rectangularly placed 
hemispheres. 
B.—Ditto, ditto, a larger form similarly magnified. 
C.—Frustulia Saxonica, magnified by the same power, and showing similar 
structure. 
D.—A small form of N. rhomboides, also magnified 800 diameters, revealing 
the same structure. : 
E. —-A very large form of NV. rhomboides from “ Cherryfield,’ America, showing 
a corresponding structure, with a magnification of 600 diameters. 
* <M. M. J.’ vol. xiv. p. 45. 
