"Cat is said to have been the first potion created specifically for witchers. The concoction allows witchers to pursue monsters into their lairs, including dark caves, ruins and crypts, because it augments vision to pick up additional wavelengths of light. To prevent blinding, the imbiber's pupils automatically narrow when in bright light. This is possibly the potion most commonly used by witchers."
Cat potion grants vision in total darkness, a very handy ability in crypts and caves, or just badly lit interiors.
Triss gives this formula at the end of the Prologue when Geralt asks her about the sorcerer who attacked the lab.
Geralt begins the game with two Cat potions as well as the formula.
Cat is one of the potions witchers use most often. After imbibing it, the witcher is able to see in total darkness and will avoid being surprised by any nocturnal creatures. The potion allows its user to see both living creatures and those made of inanimate matter, even through walls.
Cat slightly disrupts perception, however, as a result of which its user deals less damage to monsters. The Rook potion can be used to alleviate this side effect.
Potion causes minor intoxication when imbibed.
The Cat potion is recommended before entering a cave or a crypt, and before a midnight stroll through dangerous terrain.