400 
Transactions of the Society. 
If it is not small, but a large portion of the object is illuminated, 
there will be glare, and it will be quite noticeable until the aperture 
of the condenser is reduced to about three-quarters of the aperture 
of the object glass, being in some cases perceptible until the 
diaphragm of the condenser is reduced to even one-third of the 
object-glass aperture. 
For example, using an 8 mm. achromatic object glass 0*5 N.A. 
with a X50 eye-piece, which gives a field of view of 0 * 008 in., 
when the focussed image of a Pointoliglit on the object is 
0 - 00075 in. diameter there is no glare with a full aperture l’X.A. 
in condenser; with the image of the Pointolite 0‘ 00275 in. there is 
no glare; with image of the Pointolite O’ 00675 in. there is con- 
siderable glare, reducing resolution ; with ground glass giving a 
very large image, glare is excessive. 
In most cases if a ground glass or daylight is not employed the 
majority of the glare, but not the whole, disappears if the aperture 
of the condenser is not greater than three-quarters of that of the 
object glass ; and it may safely be said that for objects mounted dry 
or in styrax or realgar three-quarters of the aperture of the object 
is the largest useful cone of illumination when a large area of the 
object is illuminated, but that the whole aperture of the object 
glass can be advantageously employed if only a small area of the 
object is illuminated. 
Referring to the first experiment with a § in. object glass, with 
a X25 eye-piece, on such a diatom as Gymbella Gastraoides 
mounted in styrax, even if no substage condenser be used, but the 
light from an open sky be thrown upon the object by the concave 
mirror, a large area of the slide will be illuminated and a glare 
will make the comparatively coarse markings of this diatom very 
faint. This glare is entirely removed if a black piece of paper 
with a fine pinhole be placed immediately under the object, thus 
reducing the area of the object illuminated without otherwise 
altering the character of the illumination. 
A large number of experiments established the above con- 
ditions under which glare is met with — namely : 
The larger the cone of illuminating light, the greater the 
tendency to glare. 
The larger the area of the object illuminated, the greater the 
tendency to glare 
Glare due to light entering the observer’s eye which has not 
passed through the object is most likely to arise from reflections. 
The microscope has large numbers of lenticular surfaces, from 
four to twelve in the object glass, and four in the eye-piece ; the 
eye itself has others. The object is mounted on a glass slip with 
two flat surfaces, and is covered with a cover glass with two other 
flat surfaces. It is interesting to note that light reflected back 
from the upper surface of the eye-piece top lens and from the 
