properly calibrated monitors will show an even transition between values of each square above, with no two looking the same if values in box 15 through 18 look identical on your monitor, the photographs on this site will seem dark
tonal values can be adjusted by using the brightness and contrast buttons of your monitor grayscale values are also a direct reflection of the gamma settings of your operating system
(macintosh users set your gamma to 1.8, windows users to 2.2)