404 
Transactions of the Society. 
From the foregoing experiments we may conclude that the 
cause of the majority of glare and flooding, provided the aberrations 
of the microscope are well corrected, originates from light reflected 
backwards and forwards either along the 3x1 slip, the cover 
glass, or in the medium between the cover glass and the slip. It 
may also be increased by reflection between the front of the con- 
denser and the under-surface of the slip. Reflections from the 
surfaces of the lenses of the microscope do not appear to be 
sufficient to cause much effect with transmitted light. 
We can also gather that little perceptible glare is likely to 
occur if the object is mounted in Canada balsam, although it can 
be observed. If the object is mounted dry, in water, styrax or any 
medium that has not the same refraction as glass, there is great 
tendency to glare. 
The only way in which it can be removed is by illuminating a 
very small portion of the object if it is desired to fill the whole 
aperture of the object glass with light. 
If it is inconvenient to illuminate a small portion of the object, 
then the condenser should be stopped down so that it only fills 
about three-quarters of the aperture of the object glass ; but a 
portion of the resolving power of the object glass is sacrificed. It 
is best not to illuminate an area of the object glass larger than the 
field of view at any time. 
I am inclined to think that if the corrections of the aberrations 
of a lens are perfect, and the area of the object is not larger than 
one-fifth the field of view with a X25 eye-piece, no appreciable 
glare will be produced whatever the angle of the illumination. 
As far as I am aware this subject has not hitherto been 
investigated, and the above line of research should be carried 
considerably farther before dogmatic rules of illumination can be 
drawn up ; but until this is done no satisfactory procedure for test- 
ing microscope object glasses can be laid down. 
The foregoing results suggest that the best method of trans- 
parent illumination is to fill the aperture of the object glass with 
light with a wide angle condenser almost in focus, to cut down to 
the exact aperture of the object glass and to use something like a 
point source of light, and that where a large area of field must be 
illuminated three-quarters only of the aperture of the object should 
be filled with light and a certain amount of the resolution sacrificed. 
When the source of light is exactly in focus the resolution 
disappears, and the best resolution is obtained when the image of 
the light is almost but not quite in focus, and the resolution 
appears to be more brilliant around the margins of the image of 
the light. 
Suggestions have been put forward to explain this which have 
reference to the phase relationship of the light. The writer has 
formed the opinion that it is due to some form of glare. His 
