The Illumination of Microscope Objects. 
401 
cornea of the eye can be detected on the surface of an opaque 
ofDject on the stage of the microscope under suitable conditions, 
and 1 devised a series of experiments to ascertain in what manner 
the majority of the glare was caused. 
All the reflecting surfaces are transparent, and the reflection 
at transparent surfaces is much greater if the light is oblique. If 
light falls at normal incidence on a glass plate with a refractive 
index of 1*5, the approximate amount that is reflected at 
0° is 4 p.c. 75° is 25 p.c. 
45° „ 5 „ 85° „ 60 „ 
60° „ 9 „ 
The amount reflected is small when the incidence is direct, but 
very rapidly increases as the angle goes up beyond 60°. I there- 
fore selected a lens with an aperture of 0*65 N.A. (81°) for the 
object glass for the following experiments, and a substage con- 
denser with an angle used as an immersion of 1*3 N.A. Used 
dry it was giving a theoretical 1 N.A., in practice probably about 
0*95 N.A. An object glass with a moderate angle was therefore 
used, and a condenser which would give a much greater angle of 
illumination if desired. 
In a paper which I read on the bull’s-eye used in connexion 
with dark-ground illumination, I pointed out that as much as 
fifteen times the amount of illumination was sometimes obtained 
on an object illuminated on a dark ground due to reflections 
between the cover glass and the slip ; and this naturally suggests 
that if an object seen by transparent light has thrown upon it by 
this means a large quantity of light from above, or obliquely from 
the side, the black portions would appear grey, or would even be 
obliterated altogether. 
My first experiment was directed to, ascertain if the front flat 
surface of the object glass reflects much light down upon the 
object or the cover glass which again enters the microscope. The 
diameter of the front surface of the 8 mm. object glass employed 
is 0*25 in., the central portion only of which is used to transmit 
the light. I therefore arranged the illumination to produce glare 
both to an exaggerated extent and also to a slight extent, and 
placed an opaque stop with an aperture of 0*07 in. over the front 
of the object glass. This allowed all the light that formed the 
image to enter the instrument. I could see no difference in the 
glare in either case whether this stop was used or not, which 
showed that whatever glare may be caused by this surface is of 
little consequence, because reducing this reflecting surface to 
one-fifteenth of its area does not produce any perceptible result. 
I then unmounted a dry mount of a diatom Pleurosigma 
quadratum , and examined the glare when the diatom was on a 
