3e SRD:Illusion School
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Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, not see things that are there, hear phantom noises, or remember things that never happened. Illusions come in five types: figments, glamers, patterns, phantasms, and shadows.
Saving Throws and Illusions (Disbelief)
Creatures encountering an illusion effect usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.
A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.
A failed saving throw indicates that a character fails to notice something is amiss. A character faced with incontrovertible proof that an illusion isn't real needs no saving throw. If any viewer successfully disbelieves an illusion and communicates this fact to other viewers, each such viewer gains a saving throw with a +4 bonus.