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XVIII..—' THE ILLUMINATION OF 
MICROSCOPE OBJECTS: GLARE AND FLOODING 
WITH TRANSMITTED LIGHT. 
By Conrad Beck, C.B.E., F.R.M.S. 
(Read November 15, 1922.) 
Experienced microscopists are familiar with the effect observed 
with certain forms of illumination when, although the microscope 
in use is of the best, the object appears to be covered with a fog or 
mist, often to such an extent as to obscure the fine detail and 
sometimes to render the object almost invisible. The following 
experiment will demonstrate the effect. Examine a diatom with, 
say, a | in. object glass and a high eye-piece, use a wide angle 
substage condenser focussed upon the object, and with a flat mirror 
illuminate the condenser with daylight or light from a piece of 
illuminated ground glass, not smaller than 2 in. square, placed, 
say, 3 in. from the mirror. Open the iris diaphragm of the 
substage condenser to its full aperture, and this appearance of glare 
or flooding will render a semi-transparent diatom almost invisible. 
The same result is observed with a higher power, such as 
a J in. or ^ in., the amount of the effect varying according to the 
details of the illumination. 
The cause of this glare is that light enters the eye that has not 
passed through the object. If a painting is so hung that the glass 
which covers it reflects the light from a bright window into the 
observer s eye, he sees but little of the picture, and for the same 
reason if light enters the microscope that has not passed through the 
object, the microscopist sees but an indistinct picture of the object. 
In order to investigate the subject it is necessary to ascertain 
the conditions under which the glare occurs. First experiments 
were made with objects mounted either dry or in styrax or realgar. 
If such objects are illuminated by a very narrow-angled cone of 
light, either by the use of a flat mirror alone or by a condenser 
which is stopped down to a small angle by closing the iris 
diaphragm, little glare can be seen. The most extreme form of 
glare will be produced when a substage condenser is used with the 
iris diaphragm opened to give an illuminating cone of great angle 
and when the source of illumination is large, as, for instance, 
daylight or an illuminated ground glass. 
If the image of the source of light is very small so that only a 
small patch in the centre of the field is illuminated, there will be 
no glare even with the full aperture of the substage condenser. 
