Logarithmic that linear, it is not the amount of change, but percentage it changes by
The principle that two stimuli must differ by a constant minimum percentage (rather than a constant amount) to be perceived as different.
ex. if you 1 to 20 candles (20 percent), you'll notice rather than 1 to 120 candles (1 percent), 6 to 120 candles (20 percent)