*j?uSi. jSy Mm ^oyal Microscopical Society. 9 
from the extreme closeness of the very minute hemispheres in the 
longitudinal rows. Eound the valve, and forming an elegant 
border, are three rows of beads or hemispheres gradually decreasing 
in size — then, on the semi-diameter of the valve through the centre, 
14 rows of much smaller beads, numbering at least 80 in T-oW^h of 
an inch — and then the two " median lines," which consist of hemi- 
spheres as large as those in the outer row of the border. In the 
centre of the valve the boss or umhilicus pushes out the adjacent 
beads of the median rows into an oval form. Powell's immersion 
lens would bear with admirable effect upon this exquisite object, 
and bring out the wondrous structure which, without the aid of the 
microscope, must have remained among the invisible things of Him 
who created all things. This lens would no doubt show also an 
exactly similar structure on the still more difficult valve Amphi- 
fleura acus, the shadows of the beads being already seen as appa- 
rent lines. 
Such and so satisfactory is the work of the diatom-prism, which 
has made the microscope, old observer as I am, quite a new instru- 
ment to me. This is evident from the curious coincidence of my 
having given such a different description of rhomhoides at the last 
meeting of the Society. In then describing the markings as brought 
out by a supposed improvement of the double hemispherical con- 
denser, I used the language of the craft, and spoke of "dots as 
black as jet ; " but this mere silhouette representation of rhomhoides , 
an unnatural distortion of light and shade, I never wish to see again. 
A single sentence wiU be sufficient to describe the diatom-prism 
illumination. I place an equilateral prism below the stage of the 
microscope, and the light, either of the sun or of a lamp, after 
being totally reflected, is made to fall obliquely on the valve to be 
examined. The light of a lamp is condensed in parallel rays by 
means of a bull's-eye lens. This is all. But why never used ! Is 
it possible that, without making the trial, a supposed deficiency of 
the power of a few parallel rays could prove a bar to the experi- 
ment ? Yet it would almost seem as if such were the case, since 
Newton, Chevalier, Amici, Brewster, and Abraham have suggested 
different modes of obtaining condensed and convergent reflected 
light, and their prisms have frequently formed adjuncts for micro- 
scopical examination. But, be this as it may, the fact remains that 
we are still without any authoritative recommendation to adopt the 
method I have described. Its advantages, however, are great and 
obvious. I have no longer two suns in my firmament, shining at 
right-angles to each other, but one source of proper light properly 
placed ; and therefore, instead of the false appearance of lines and 
striaB, rectilineal and oblique under low powers, and of hexagons 
and other fancies, under high powers. I see what really does exist, 
viz. a series of beautiful hemispheres placed in their due order on 
