if the light isnt sufficient on a microscope how do you fix it

There could be a variety of solutions to this because there could be a variety of reasons as to why the light through the eyepiece is poor. Often, the fix is fairly simple.
The objective lens might be dirty. The dust and small particulate on the lens will obstruct some of the light, so simply cleaning it with lens paper might solve the problem.
The sample that you are trying to view might not be thin enough to allow enough light to pass through. In this case, you will need to mount another sample that is thin enough to allow for enough light penetration.
It's possible that the sample is the correct thickness and the objectives are clean. The problem might be the iris diaphragm. Rotate it to a setting that allows for more light to illuminate the sample.
Light pollution from the room or open windows could adversely affect what you are trying to view as well.
Finally, the problem might be with the light source itself. The bulb could be bad and not using the full voltage it is being given. A newer or brighter bulb is a relatively simple fix. Finally, you might have a microscope that has a rheostat. This allows you to alter the voltage going to the light source.
https://science.howstuffworks.com/light-microscope2.htm

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