At What Tilt Does a Hung Frame Read as Crooked?: A Method-of-Constant-Stimuli Threshold Study of 312 Observers, and the Levelling Tolerance Adopted From ItResolving to At What Tilt Does a Hung Frame Read as Crooked?: A Method-of-Constant-Stimuli Threshold Study of 312 Observers, and the Levelling Tolerance Adopted From It…