What you want is a mixture of levels adjustment and curves adjustment, which will give you the following result (left side: original image, right side: improved version):
I used both methods to improve the contrast in your image, which, to human eyes, makes the image sharper as well. Since I don't know what you're looking for in these images (which shades and nuances are important), I applied them very carefully; you can use them much more radically to put more contrast in there (at the expense of losing some nuances in the middle tones).
While the levels + curves adjustments effectively do the same thing as the sledgehammer method of "enhance contrast + brightness", they give you much more control over what you're doing, so you can put bright and dark tones exactly where they belong.
For instructions on how to use both tools in Photoshop, just follow this links:
For anyone else reading this, please note that both tools are also available in Open Source Software such as Gimp, so there's no need to buy expensive proprietary software if you don't want to. My two versions above were made using Gimp.