You need a reducer. There are three types.
Originally Posted by Pandora
1. Subtractive, which remove image silver in equal density increments from from the highlights and shadows. Good for negatives that are overexposed and does not cause a change in contrast.
2. Proportional, which removes image silver so that the density is decreased prportionally to original density. Good for correcting for over-development.
3. Super Proportional, which remvoes image silver so tht the denser parts of the image lose more silver than the areas of lesser density. Also good for correcting for over-development, but causes a shouldering of highlight values.
Reducers are a bit tricky but if used with care can give excellent results. Basically you need to decide which of the three typs of correction is needed for your negatives, and then chose the proper reducer.