Correcting the problem with the highlights in the image is just as easy as correcting the shadows. Simply click on the white point slider and drag it towards the left just to the point where the right edge of the histogram begins:

As you drag the slider, you'll see the bright areas in the image becoming gradually brighter. With the white point slider moved to the right edge of the histogram, the pixels that were a light shade of gray a moment ago are forced to pure white, causing all of the light areas in the image to become lighter in the process. Here's my image after setting the new white point. The highlights are now nice and bright, and the overall image contrast has been greatly improved from how it looked originally:

Once again, if we look to the Histogram palette, we can see the effects of the changes we've made. The right side of the histogram now extends all the way to the right edge, telling us that our highlights are now nice and bright. And by forcing the highlights to white, we've stretched out our image information even further, losing more detail at various brightness levels and creating even more of a comb-like effect in the histogram:

As a side note, if you've been wondering why my histogram is showing a tall spike near the right edge, it's because this particular photo that I'm working with consists mainly of a light blue lake and a light blue sky. In other words, it's made up mostly of light blue, which means the majority of the pixels in the image have a similar brightness value. Since the histogram shows us a comparison of the various brightness levels in the image, having so many pixels sharing a similar brightness value is causing that level to tower over the others. All photos are different, and if you're following along with your own image, your histogram will undoubtedly look different from mine.

At this point, we've successfully lightened our shadows and brightened our highlights and the image is looking much better. However, one problem you may run into after adjusting the black and white levels is that the overall image can still appear either too bright or too dark. To fix that, we simply need to adjust the midtone slider. Dragging the midtone slider towards the left will brighten the image in the midtones, while dragging it towards the right will darken the midtones. It's important to note that the midtone slider does not affect the black or white points. Only the brightness levels between black and white are affected.

In my case, I think my image still looks a little washed out, so I'll click on the midtone slider and drag it a short way towards the right to darken it. Unlike the black and white points, there is no specific setting for the midtone slider. How far you drag the slider is entirely up to you and how light or dark you think the midtones need to be, so you'll want to keep an eye on your image in the document window so you can judge the results. Here, I'm dragging the midtone slider a little towards the right. You don't usually need to drag it very far to get the results you need:

Let's check out the Histogram palette one last time. If you look closely, you'll notice that the left side of the histogram now seems to be missing fewer brightness levels than the right side does. That's because by darkening the midtones, we've taken image information from the lighter tonal values and pushed it into the darker values. This filled up some of the missing shadow areas but stretched out the lighter areas even further:

Again, even though the histogram now looks like we're missing quite a bit of detail (which we are), what Photoshop sees and what our eyes see are two different things. Unless you've lost so much detail that you can actually see areas of color or brightness values missing in your image (known as banding or posterizing), you don't need to worry about how bad the histogram looks. All that matters is how the image looks. And with that, here for comparison is my original image once again:

And here, after setting new black and white points and adjusting the midtones in the Levels dialox box (which you can now close out of by clicking the OK button to accept the changes you''ve made) is my final result:

And there we have it! That's how easy it is to correct overall tonal problems in an image with Levels in Photoshop!