The instrument for measuring atmospheric pressure, the barometer, is calibrated to read zero when there is a complete vacuum; the pressure indicated by the instrument is therefore called absolute pressure. The term pressure gauge is commonly applied to the other instruments used for measuring pressure.
They see what's already there in a different way, and most of the time, they're aware that their altered perceptions are caused by the drug. After taking LSD, the effects -- known as a trip -- usually start within an hour and can last up to 12 hours, with a peak about halfway through the experience.
An average single dose of LSD during the 1960s was between 100 and 150 micrograms, a tiny amount roughly equal to one-tenth the weight of a grain of sand. Today, a typical single dose of LSD is as low as 25-50 micrograms. Threshold effects can be felt with as little as 20 micrograms.