Correcting the Exposure and Color Balance When correcting for exposure and color it s helpful to have the original picture in font of you. Since we do not have the original picture, use your best judgment. ...

Use the Zoom Tool to make the image 100%. Click on the Zoom Tool, then click on the image. Every time you click with the Zoom Tool the image gets a little larger. To reduce the size of the image, hold down the ALT key (note that a minus sign appears in the center of the magnifying glass), and Click to reduce the size of the image.

Adjust brightness by clicking on the pictures labeled Lighter and Darker on the right-hand side. Keep comparing the Current Pick (top row) to the reference picture.

Brightness/Contrast Increasing contrast makes the light areas lighter and the dark areas darker; decreasing it makes the light areas darker and the dark areas lighter.
...
Source: library.med.utah.edu