There are 20 bars above ranging from black to white.  If you can not see all 20 bars, you need to raise the brightness of your monitor and increase the contrast.

If you are unable to see an image in the darkness, adjust your monitor's contrast and brightness until you can see some detail in the image.

The above image is a darkened version of this image.

The views between the two images are very slightly different, but are close enough in similarity to provide a reference for what you are expected to see in the above image.