130 MAX SCHULTZE, ON THE DIATOM-VALVE. 
Amongst other observers who are inclined to adopt this view 
may be cited Mr. G. Norman, of Hull, than whom few pos- 
sess a more intimate knowledge of diatoms.* It is curious, 
in this state of the question, that the method of observation 
proposed by Welcker, though not for this special purpose, 
should not have been employed. Welcker states, as a means 
of distinguishing, in all transparent objects, superficial eleva- 
tions from depressions, that elevations appear brightest when 
the body of the microscope is raised, whilst depressions, on 
the contrary, are brightest when it is depressed. If we start, 
then, with the body of the microscope at a medium height, 
or lowering it gradually upon the object from a height alto- 
gether out of focus, in this case elevations on the surface will 
first appear as bright points on a dark ground, and depres- 
sions as dark points on a bright ground, until, as we con- 
tinue to lower the tube, the image in either case is reversed. 
It is only requisite to observe that the object should be 
mounted in a medium having a lower index of refraction 
than itself. 
The reason of this is, as stated by Welcker, because the 
elevations act as convex and the depressions as concave 
lenses, the bright points representing the focus of each re- 
spectively, and consequently corresponding in the one case io 
the summit of the elevation and in the other to the bottom 
of the depression. 
In proceeding to apply this rule to diatoms, it will be most 
convenient to select dry preparations, in which the first con- 
dition, viz., that objects should be placed in a less refractive 
medium, is fulfilled, and in which also, as is well known, the 
superficial markings are most readily made out. But even 
with a magnifying power of 1000 diameters and more, and 
with excellent lenses, capable of distinctly brmging out the 
markings on P. angulatum with direct illumination, we shall 
soon be convinced that the subject, even as regards the more 
easy images of P. balticum, attenuatum, and hippocampus, still 
presents unforeseen and almost insuperable difficulties. 
An indispensable precaution to be taken to ensure success 
in this inquiry is that an individual point in the marking 
should be so distinctly fixed upon that it may be recognised 
under various alterations of the focus. But the points and 
series of points of the diatoms above named are so closely 
approximated that this object demands considerable effort 
and skill. The author thinks he has been successful in the 
attempt in the case of P. balticum, and has satisfied himself 
* «Quart, Journ, Mic, 8c.,’ July, 1862, p, 212. 
