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Transactions of the Society. 
thin cover glass, first with the cover glass above the diatom, and 
then with the cover glass below the diatom. 1 n both cases, a ground 
glass 2 in. square was used, at a distance from the condenser of 
about 8 in. (focal length of condenser 0 • 33 in.), and the glare was so 
great if the aperture was opened even slightly beyond that of the 
object glass, that all resolution was destroyed. If the ground 
glass was removed and the Pointolite was almost focussed, full 
resolution was obtained with the condenser opened to 1 N.A. 
I then broke up the cover glass and hunted around the edges 
of the broken pieces until I found a diatom partly projecting over 
the edge, hanging in the air so that it had no cover glass above or 
below it. Then arranging the illumination to give glare, the 
resolution of the portion hanging in mid-air was perfect and free 
from glare with the full aperture of the condenser, while the 
portion of the same diatom which was on the cover glass showed 
no markings whatever, until by closing the diaphragm of the con- 
denser to about the aperture of the object glass the markings on 
both parts of the diatom were equally well resolved. 
This established the fact that reflection at the cover glass 
produced glare. 
I then took a diatom on a cover glass, and put the lower side 
of the cover glass in immersion contact with the substage con- 
denser, and found that although there was a very slight glare it 
was scarcely noticeable ; and if the immersion fluid, the front lens 
of the condenser, and the cover glass had all been of exactly the 
same refractive index, I do not think any glare whatever would 
have been visible. 
To carry the matter to a more definite conclusion I corrected 
the achromatic substage condenser so that it was strictly corrected 
for a slip 0 * 042 in. thick for a distance of 8 in. To do this I 
placed a microscopic pinhole 8 in. from the condenser, and set the 
‘Correction collar of the substage condenser to correct all aberra- 
tions. I then placed a small circular opaque spot at 8 in. from 
the condenser, and focussed a perfect image of this spot upon an 
object which was mounted on the 0*042 in. thick slip. The image 
of the black disc, which occupied about one -fifth of the field of 
view, was very perfect, and showed no shade of greyness round its 
edges, even if the full aperture of the condenser was used. I then 
placed a ground glass immediately behind the disc, and examined 
the effect of glare in rendering the black disc grey, or even white. 
The objects selected were on slips that were O' 041 in., 
0*0415 in., 0*042 in., 0*0425 in. thick, and the condenser was 
recorrected for each slip to allow for the above small differences. 
Three of the objects were diatoms mounted respectively in air, 
styrax and realgar; the fourth object was a malaria parasite 
mounted in balsam ; and a fifth was a plain 3x1 glass slip. 
The results were as follows : — 
