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appear to form patterns like the Gothic quatrefoils, and look like 
holes when the light passes straight through them. 
To see this structure well the mirror must he inclined so that 
the light given by Mr. Wenham’s apparatus all passes a little 
askew through the spherules. When, through refraction or reflexion, 
any light passes straight, or nearly straight, through the larger 
spherules, they look like holes through which a glare penetrates, 
and all minutiae of structure disappear. A slight change in the 
angle of the mirror, or of the position of the Wenham illuminator 
effected by partial rotation, will leave the larger spherules in a quiet 
glow, as if they were incompletely transparent, while the smaller 
spherules of the radial bands glisten a pearly lustre. A 1th of Beck’s 
with a deep eye-piece is sufficient to show this structure, but an 
|th will do it better, and an excellent effect was obtained with 
Powell and Lealand’s new pattern and dry front. 
Good specimens of Eupodiscus Argus should be examined as 
uncovered objects both with Mr. Wenham’s apparatus and with that 
devised for opaque objects under high powers by Professor Smith. 
The writer has hitherto had no opportunity of trying these plans. 
With regard to Isthmia enervis, the ‘ Micrographic Dictionary ’ 
describes the valves as “ without linear thickenings, uniformly 
covered with depressions,” and the valves of the genus generally are 
spoken of as having “ depressions so large as to produce a distinct 
reticular or cellular appearance when viewed by ordinary illumina- 
tion.” This last remark is quite correct, and the “ distinctness ” 
of the appearance has caused it to pass unquestioned. 
In a last edition of ‘ Pritchard’s Infusoria ’ the valves of Isthmia 
enervis are described as having “ large, somewhat quadrate cellules,” 
which corresponds with the appearances ordinarily seen. In the 
same work, at page 42, is a quotation from ‘ Silliman’s J ournal,’ 
describing experiments of Professor Bailey, who viewed diatom 
valves with the microscope while they were being acted on with 
dilute hydrofluoric acid. “ In Isthmia,” he said, ££ the spots on the 
surface which appear like granular projections, are in reality thin 
portions of the shell, and under the action of the acid they soon 
become holes.” It is obvious that the parts which dissolve first 
must either be the thinnest or so constructed as to present the 
greatest surface to the acid. Professor Bailey’s observations must 
therefore guide us in deciding which of several appearances the 
valves can be made to assume under high powers and various 
modes of illumination. 
The writer has not yet been fortunate enough to find specimens 
of Isthmia so situated as to show in the best manner with Mr. 
Wenham’s illuminator, but with a fine fth objective and that 
apparatus the simple cellular or reticulated appearance disappears, 
and great numbers of minute spherules at different levels can be 
