Darkening pixel values

I am trying to quantitate data on some black and white microscope pictures I took, but the images are too bright in parts and I need to darken them down. Is there a way that I can use photoshop to darken certain pixel values within the photo? I have tried adjusting the levels in the picture so that the greys are limited, and I have tried changing the greyscale, but to no avail. I need to make it so that two images have the same pixel values so that they can be calculated evenly. Can anyone help?

There are many ways to do this .

1.) Use Levels (with values) but be careful.

2.) Curves (The best way) (but with values)

3.) With hue/saturation (it has a lightness/darkness) and values

and several other ways. You can also put both images into the same photoshop file and put a level adjuster over the layers and no matter what layer you are on it has the same value. very helpful.

I hope this helps!