Keith,
Could be lamp, eyepieces  or other optics in the microscope.
The easiest thing to start with is to change the bulb and make sure that
it is aligned correctly.  You can try illuminating from above. A ring
illuminator would be ideal but if not then try an illuminator from one
side at a time at the same distance from column. If  problem persists then
probably a microscope problem.
It is possible that the left eyepiece has a problem. Have you tried
switching eyepieces to check this?  (I am assuming that you have matching
eyepieces?)
A dirty prism in the column or else the prism is misaligned is also a
possibility.
depending on the microscope, if you can take the turret holding the
eyepieces off then you can see the prism and any intermediate lenses and
determine if they are dirty.  As the specimen remains as a concentric
image when zooming is probably not a misaligned prism.
You can even out illumination considerably by using frosted glass.
Frosted glass can be used with the frosted side closest to the lamp so
that focus will not be a problem and the pattern not visible.
Call me if you would like to discuss any of these.
Barry

Barry

Keith Ryan wrote:

> Hi All
>
> I am trying to help a student who has been bought a 10-40x stereo
> zoom microscope with some image recording/analysis equipment.  The
> microscope is totally unbranded - nothing on the body or the eyepieces
> (except for magnification).
>
> The problem is an apparent mismatch of the lamp intensity in the
> eyepieces. In the right ocular, the illumination is brighter
> centrally, and it zooms up concentrically - that's fine.  In the left
> ocular, the brighter central zone zooms up to go out of the field of
> view, to the left.  A centred specimen zooms up concentrically in both
> oculars.  Any suggestions?  Is it a phenomenon of the different height
> of the bulb, which is directly below the stage, and the specimen?  The
> problem is apparently alleviated by putting a piece of velin tissue
> (thin paper) under the specimen.  But this introduces a patterned
> background.
>
> Keith Ryan
> Marine Biological Association
> Plymouth UK