Font of Power
Source: Advanced Class Guide, p. 77
An eldritch font gains one additional spell slot for each level of arcanist spell she can cast. However, the number of spells of each level that she can prepare decreases by 1. If this reduces her spells prepared for a level to 0, she still gains spell slots of that level, which can be used to fuel her arcane reservoir (using the consume spells class feature) or to cast spells using metamagic feats.